Dark matter#Direct detection

The hypothesis of dark matter has an elaborate history.{{cite journal |last1=de Swart |first1=J.G. |last2=Bertone |first2=G. |last3=van Dongen |first3=J. |year=2017 |title=How dark matter came to matter |journal=Nature Astronomy |volume=1 |issue=59 |page=59 |arxiv=1703.00013 |bibcode=2017NatAs...1E..59D |doi=10.1038/s41550-017-0059 |s2cid=119092226 }}

Wm. Thomson, Lord Kelvin, discussed the potential number of stars around the Sun in the appendices of a book based on a series of lectures given in 1884 in Baltimore. He inferred their density using the observed velocity dispersion of the stars near the Sun, assuming that the Sun was 20–100 million years old. He posed what would happen if there were a thousand million stars within 1 kiloparsec of the Sun (at which distance their parallax would be 1 milli-arcsecond). Kelvin concluded

Many of our supposed thousand million stars – perhaps a great majority of them – may be dark bodies.{{cite book |last=Thompson |first=((W., Lord Kelvin)) |author-link=William Thomson, 1st Baron Kelvin |year=1904 |title=Baltimore Lectures on Molecular Dynamics and the Wave Theory of Light |publisher=C.J. Clay and Sons |place=London, UK |page=274 |url=https://babel.hathitrust.org/cgi/pt?id=ien.35556038198842&view=1up&seq=304 |via=hathitrust.org }}{{cite magazine |title=A history of dark matter |date=2017-02-03 |df=dmy-all |magazine=Ars Technica |url=https://arstechnica.com/science/2017/02/a-history-of-dark-matter/ |access-date=8 February 2017 |lang=en-us}}

In 1906, Henri Poincaré used the French term [{{Lang|fr|matière obscure}}] ("dark matter") in discussing Kelvin's work.{{cite journal |last1=Poincaré |first1=H. |author-link=Henri Poincaré |year=1906 |title=La Voie lactée et la théorie des gaz |journal=Bulletin de la Société astronomique de France |volume=20 |pages=153–165 |url=https://babel.hathitrust.org/cgi/pt?id=uiug.30112110949630&view=1up&seq=171 |trans-title=The Milky Way and the theory of gases |language=fr}} He found that the amount of dark matter would need to be less than that of visible matter, incorrectly, it turns out.

The second to suggest the existence of dark matter using stellar velocities was Dutch astronomer Jacobus Kapteyn in 1922.{{cite journal |first=J.C. |last=Kapteyn |author-link=Jacobus Kapteyn |year=1922 |title=First attempt at a theory of the arrangement and motion of the sidereal system |journal=Astrophysical Journal |volume=55 |pages=302–327 |bibcode=1922ApJ....55..302K |doi=10.1086/142670 |quote=It is incidentally suggested when the theory is perfected it may be possible to determine the amount of dark matter from its gravitational effect. {{grey|[emphasis in original]}} }}{{cite conference |last=Rosenberg |first=Leslie J. |date=30 June 2014 |title=Status of the Axion Dark-Matter Experiment (ADMX) |conference=10th PATRAS Workshop on Axions, WIMPs and WISPs |page=2 |url=http://indico.cern.ch/event/300768/session/0/contribution/30/attachments/566901/780884/Rosenberg-Patras_30jun14.pdf |url-status=live |archive-url=https://web.archive.org/web/20160205163816/http://indico.cern.ch/event/300768/session/0/contribution/30/attachments/566901/780884/Rosenberg-Patras_30jun14.pdf |archive-date=2016-02-05 |conference-url=http://axion-wimp2014.desy.de }}

A publication from 1930 by Swedish astronomer Knut Lundmark points to him being the first to realise that the universe must contain much more mass than can be observed.{{cite journal |last=Lundmark |first=K. |author-link=Knut Lundmark |date=1930-01-01 |title=Über die Bestimmung der Entfernungen, Dimensionen, Massen, und Dichtigkeit fur die nächstgelegenen anagalacktischen Sternsysteme |lang=de |trans-title=On determination of distances, dimensions, masses, and densities for the nearest non-galactic star systems |journal=Meddelanden Fran Lunds Astronomiska Observatorium |volume=125 |pages=1–13 |bibcode=1930MeLuF.125....1L |url=https://ui.adsabs.harvard.edu/abs/1930MeLuF.125....1L }} Dutch radio astronomy pioneer Jan Oort also hypothesized the existence of dark matter in 1932.{{cite journal |last=Oort |first=J.H. |author-link=Jan Oort |year=1932 |title=The force exerted by the stellar system in the direction perpendicular to the galactic plane and some related problems |journal=Bulletin of the Astronomical Institutes of the Netherlands |volume=6 |pages=249–287 |bibcode=1932BAN.....6..249O }}{{cite web |title=The hidden lives of galaxies: Hidden mass |website=Imagine the Universe |publisher=NASA / GSFC |place=Greenbelt, MD |url=http://imagine.gsfc.nasa.gov/teachers/galaxies/imagine/hidden_mass.html }} Oort was studying stellar motions in the galactic neighborhood and found the mass in the galactic plane must be greater than what was observed, but this measurement was later determined to be incorrect.{{cite journal |last1=Kuijken |first1=K. |last2=Gilmore |first2=G. |date=July 1989 |journal=Monthly Notices of the Royal Astronomical Society |volume=239 |issue=2 |pages=651–664 |bibcode=1989MNRAS.239..651K |title=The Mass Distribution in the Galactic Disc – Part III – the Local Volume Mass Density |doi=10.1093/mnras/239.2.651 |doi-access=free }}

In 1933, Swiss astrophysicist Fritz Zwicky studied galaxy clusters while working at Caltech and made a similar inference.{{cite journal |last=Zwicky |first=F. |author-link=Fritz Zwicky |date=1933 |title=Die Rotverschiebung von extragalaktischen Nebeln |trans-title=The red shift of extragalactic nebulae |journal=Helvetica Physica Acta |volume=6 |pages=110–127 |bibcode=1933AcHPh...6..110Z }}{{efn|

"Um, wie beobachtet, einen mittleren Dopplereffekt von 1000 km/sek oder mehr zu erhalten, müsste also die mittlere Dichte im Comasystem mindestens 400 mal grösser sein als die auf Grund von Beobachtungen an leuchtender Materie abgeleitete. Falls sich dies bewahrheiten sollte, würde sich also das überraschende Resultat ergeben, dass dunkle Materie in sehr viel grösserer Dichte vorhanden ist als leuchtende Materie."{{rp|style=ama|p=125}}

: [In order to obtain an average Doppler effect of 1000 km/s or more, as observed, the average density in the Coma system would thus have to be at least 400 times greater than that derived on the basis of observations of luminous matter. If this were to be confirmed, the surprising result would then follow that dark matter is present in very much greater density than luminous matter.]

}}{{cite journal |last=Zwicky |first=Fritz |author-link=Fritz Zwicky |date=1937 |title=On the Masses of Nebulae and of Clusters of Nebulae |journal=The Astrophysical Journal |volume=86 |pages=217–246 |bibcode=1937ApJ....86..217Z |doi=10.1086/143864 |doi-access=free}} Zwicky applied the virial theorem to the Coma Cluster and obtained evidence of unseen mass he called {{Lang|de|dunkle Materie}} ('dark matter'). Zwicky estimated its mass based on the motions of galaxies near its edge and compared that to an estimate based on its brightness and number of galaxies. He estimated the cluster had about 400 times more mass than was visually observable. The gravity effect of the visible galaxies was far too small for such fast orbits, thus mass must be hidden from view. Based on these conclusions, Zwicky inferred some unseen matter provided the mass and associated gravitational attraction to hold the cluster together.Some details of Zwicky's calculation and of more modern values are given in {{cite report |first=M. |last=Richmond |date=c. 1999 |title=Using the virial theorem: The mass of a cluster of galaxies |type=lecture notes |series=Physics 440 |publisher=Rochester Institute of Technology |place=Rochester, NY |url=http://spiff.rit.edu/classes/phys440/lectures/gal_clus/gal_clus.html |via=spiff.rit.edu |access-date=10 July 2007}} Zwicky's estimates were off by more than an order of magnitude, mainly due to an obsolete value of the Hubble constant;{{cite book |first=Katherine |last=Freese |year=2014 |title=The Cosmic Cocktail: Three parts dark matter |publisher=Princeton University Press |isbn=978-1-4008-5007-5 |url={{google books |plainurl=y |id=c2B8AgAAQBAJ}} }} the same calculation today shows a smaller fraction, using greater values for luminous mass. Nonetheless, Zwicky did correctly conclude from his calculation that most of the gravitational matter present was dark. However, unlike modern theories, Zwicky considered "dark matter" to be non-luminous ordinary matter.{{rp|III.A}}

Further indications of mass-to-light ratio anomalies came from measurements of galaxy rotation curves. In 1939, H.W. Babcock reported the rotation curve for the Andromeda nebula (now called the Andromeda Galaxy), which suggested the mass-to-luminosity ratio increases radially.{{cite journal |last=Babcock |first=H.W. |author-link=Horace W. Babcock |year=1939 |title=The rotation of the Andromeda Nebula |journal=Lick Observatory Bulletin |volume=19 |pages=41–51 |bibcode=1939LicOB..19...41B |doi=10.5479/ADS/bib/1939LicOB.19.41B |doi-access=free }} He attributed it to either light absorption within the galaxy or modified dynamics in the outer portions of the spiral, rather than to unseen matter. Following Babcock's 1939 report of unexpectedly rapid rotation in the outskirts of the Andromeda Galaxy and a mass-to-light ratio of 50; in 1940, Oort discovered and wrote about the large non-visible halo of NGC 3115.{{cite journal |last=Oort |first=J.H. |author-link=Jan Oort |date=April 1940 |title=Some problems concerning the structure and dynamics of the galactic system and the elliptical nebulae NGC 3115 and 4494 |journal=The Astrophysical Journal |volume=91 |issue=3 |pages=273–306 |bibcode=1940ApJ....91..273O |doi=10.1086/144167 |hdl=1887/8533 |hdl-access=free |url=https://openaccess.leidenuniv.nl/bitstream/handle/1887/8533/008_653_032.pdf?sequence=1 |via=leidenuniv.nl }}

The hypothesis of dark matter largely took root in the 1970s. Several different observations were synthesized to argue that galaxies should be surrounded by halos of unseen matter. In two papers that appeared in 1974, this conclusion was drawn in tandem by independent groups: in Princeton, New Jersey, by Jeremiah Ostriker, Jim Peebles, and Amos Yahil, and in Tartu, Estonia, by Jaan Einasto, Enn Saar, and Ants Kaasik.{{cite journal |last1=de Swart |first1=Jaco |date=1 August 2024 |title=Five decades of missing mass |journal=Physics Today |volume=77 |pages=34–43 | doi=10.1063/pt.ozhk.lfeb |doi-access=free }}

One of the observations that served as evidence for the existence of galactic halos of dark matter was the shape of galaxy rotation curves. These observations were done in optical and radio astronomy. In optical astronomy, Vera Rubin and Kent Ford worked with a new spectrograph to measure the velocity curve of edge-on spiral galaxies with greater accuracy.{{cite news |last=Overbye |first=D. |author-link=Dennis Overbye |date=27 December 2016 |title=Vera Rubin, 88, dies; opened doors in astronomy, and for women |type=obituary |newspaper=The New York Times |url=https://www.nytimes.com/2016/12/27/science/vera-rubin-astronomist-who-made-the-case-for-dark-matter-dies-at-88.html |access-date=27 December 2016 }}{{cite web |title=First observational evidence of dark matter |website=Darkmatterphysics.com |url=http://www.darkmatterphysics.com/Galactic-rotation-curves-of-spiral-galaxies.htm |access-date=6 August 2013 |archive-url=https://web.archive.org/web/20130625183052/http://www.darkmatterphysics.com/Galactic-rotation-curves-of-spiral-galaxies.htm |archive-date=25 June 2013}}{{cite journal |last1=Rubin |first1=V.C. |author1-link=Vera Rubin |last2=Ford |first2=W.K. Jr. |author2-link=Kent Ford (astronomer) |date=February 1970 |title=Rotation of the Andromeda nebula from a spectroscopic survey of emission regions |journal=The Astrophysical Journal |volume=159 |pages=379–403 |bibcode=1970ApJ...159..379R |doi=10.1086/150317 |s2cid=122756867 }}

At the same time, radio astronomers were making use of new radio telescopes to map the 21 cm line of atomic hydrogen in nearby galaxies. The radial distribution of interstellar atomic hydrogen (H I region) often extends to much greater galactic distances than can be observed as collective starlight, expanding the sampled distances for rotation curves – and thus of the total mass distribution – to a new dynamical regime. Early mapping of Andromeda with the {{convert|300|ft|m|adj=mid}} telescope at Green Bank{{cite journal |last1=Roberts |first1=Morton S. |date=May 1966 |title=A high-resolution 21 cm hydrogen-line survey of the Andromeda nebula |journal=The Astrophysical Journal |volume=159 |pages=639–656 |bibcode=1966ApJ...144..639R |doi=10.1086/148645}} and the {{convert|250|ft|m|adj=mid}} dish at Jodrell Bank{{cite journal |last1=Gottesman |first1=S. T. |last2=Davies |first2=Rod D. |author-link2=Rod Davies |last3=Reddish |first3=Vincent Cartledge |author-link3=Vincent Cartledge Reddish |date=1966 |title=A neutral hydrogen survey of the southern regions of the Andromeda nebula |journal=Monthly Notices of the Royal Astronomical Society |volume=133 |issue=4 |pages=359–387 |bibcode=1966MNRAS.133..359G |doi=10.1093/mnras/133.4.359 |doi-access=free}} already showed the H{{sup|{{math|I}}}} rotation curve did not trace the decline expected from Keplerian orbits.

As more sensitive receivers became available, Roberts & Whitehurst (1975){{cite journal |last1=Roberts |first1=Morton S. |date=October 1975 |title=The rotation curve and geometry of M 31 at large galactocentric distances |journal=The Astrophysical Journal |volume=201 |pages=327–346 |bibcode=1975ApJ...201..327R |doi=10.1086/153889}} were able to trace the rotational velocity of Andromeda to 30 kpc, much beyond the optical measurements. Illustrating the advantage of tracing the gas disk at large radii; that paper's Figure 16 combines the optical data (the cluster of points at radii of less than 15 kpc with a single point further out) with the H{{sup|{{math|I}}}} data between 20 and 30 kpc, exhibiting the flatness of the outer galaxy rotation curve; the solid curve peaking at the center is the optical surface density, while the other curve shows the cumulative mass, still rising linearly at the outermost measurement. In parallel, the use of interferometric arrays for extragalactic H{{sup|{{math|I}}}} spectroscopy was being developed. Rogstad & Shostak (1972) published H{{sup|{{math|I}}}} rotation curves of five spirals mapped with the Owens Valley interferometer; the rotation curves of all five were very flat, suggesting very large values of mass-to-light ratio in the outer parts of their extended H{{sup|{{math|I}}}} disks.{{cite journal |last1=Rogstad |first1=David H. |last2=Shostak |first2=G. Seth |author-link2=Seth Shostak |date=September 1972 |title=Gross properties of five Scd galaxies as determined from 21 centimeter observations |journal=The Astrophysical Journal |volume=176 |pages=315–321 |bibcode=1972ApJ...176..315R |doi=10.1086/151636}} In 1978, Albert Bosma showed further evidence of flat rotation curves using data from the Westerbork Synthesis Radio Telescope.{{cite thesis |last=Bosma |first=A. |date=1978 |title=The distribution and kinematics of neutral hydrogen in spiral galaxies of various morphological types |degree=Ph.D. |publisher=Rijksuniversiteit Groningen |url=http://nedwww.ipac.caltech.edu/level5/March05/Bosma/frames.html}}

In 1978, Steigman et al.{{cite journal |last=Gunn |first=J. E. |last2=Lee |first2=B. W. |last3=Lerche |first3=I. |last4=Schramm |first4=D. N. |last5=Steigman |first5=G. |date=Aug 1978 |title=Some astrophysical consequences of the existence of a heavy stable neutral lepton. |url=https://ui.adsabs.harvard.edu/abs/1978ApJ...223.1015G/abstract |journal=The Astrophysical Journal |language=en |volume=223 |pages=1015–1031 |doi=10.1086/156335 |issn=0004-637X}} presented a study that extended earlier cosmological relic-density calculations to any hypothetical stable, electrically neutral, weak-scale lepton, showing how such a particle's abundance would "freeze out" in the early Universe and providing analytic expressions that linked its mass and weak interaction cross-section to the present-day matter density. By decoupling the analysis from specific neutrino properties and treating the candidate generically, the authors set out a framework that later became the standard template for weakly interacting massive particles (WIMPs){{cite book |last=Tan |first=Chung-i |url=https://www.google.com.br/books/edition/Particles_Strings_And_Supernovae_Proceed/CtBKDwAAQBAJ |title=Particles, Strings And Supernovae - Proceedings Of Theoretical Advanced Study Institute In Elementary Particle Physics (In 2 Volumes) |last2=Jevicki |first2=Antal |date=1989-05-01 |publisher=World Scientific |isbn=978-981-4590-77-8 |pages=191 |language=en}} and for comparing particle-physics models with cosmological constraints. Though subsequent work has refined the methodology and explored many alternative candidates, this paper marked the first explicit, systematic treatment of dark matter as a new particle species beyond the Standard Model.{{citation |last=Mambrini |first=Yann |title=Introduction |date=2021 |work=Particles in the Dark Universe: A Student’s Guide to Particle Physics and Cosmology |pages=1–22 |editor-last=Mambrini |editor-first=Yann |url=https://link.springer.com/chapter/10.1007/978-3-030-78139-2_1 |access-date=2025-04-26 |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-030-78139-2_1 |isbn=978-3-030-78139-2}}

By the late 1970s the existence of dark matter halos around galaxies was widely recognized as real, and became a major unsolved problem in astronomy.

A stream of observations in the 1980–1990s supported the presence of dark matter. {{harvp|Persic|Salucci|Stel|1996}} is notable for the investigation of 967 spirals.{{cite journal |first1=Massimo |last1=Persic |first2=Paolo |last2=Salucci |first3=Fulvio |last3=Stel |year=1996 |title=The universal rotation curve of spiral galaxies — I. The dark matter connection |journal=Monthly Notices of the Royal Astronomical Society |volume=281 |issue=1 |pages=27–47 |doi= 10.1093/mnras/278.1.27 |doi-access=free |arxiv=astro-ph/9506004 |bibcode= 1996MNRAS.281...27P }} The evidence for dark matter also included gravitational lensing of background objects by galaxy clusters,{{rp|style=ama|pp= 14–16}} the temperature distribution of hot gas in galaxies and clusters, and the pattern of anisotropies in the cosmic microwave background.

According to the current consensus among cosmologists, dark matter is composed primarily of some type of not-yet-characterized subatomic particle.{{cite journal |last1=Copi |first1=C.J. |last2=Schramm |first2=D.N. |last3=Turner |first3=M.S. |year=1995 |title=Big-Bang nucleosynthesis and the baryon density of the universe |journal=Science |volume=267 |issue=5195 |pages=192–199 |arxiv=astro-ph/9407006 |bibcode=1995Sci...267..192C |doi=10.1126/science.7809624 |pmid=7809624 |s2cid=15613185 |url=https://cds.cern.ch/record/265576 }}{{cite journal |last=Bergstrom |first=L. |year=2000 |title=Non-baryonic dark matter: Observational evidence and detection methods |journal=Reports on Progress in Physics |volume=63 |issue=5 |pages=793–841 |arxiv=hep-ph/0002126 |bibcode=2000RPPh...63..793B |doi=10.1088/0034-4885/63/5/2r3 |s2cid=119349858 }}

The search for this particle, by a variety of means, is one of the major efforts in particle physics.{{cite journal |last1=Bertone |first1=G. |last2=Hooper |first2=D. |last3=Silk |first3=J. |year=2005 |title=Particle dark matter: Evidence, candidates, and constraints |journal=Physics Reports |volume=405 |issue=5–6 |pages=279–390 |arxiv=hep-ph/0404175 |bibcode=2005PhR...405..279B |doi=10.1016/j.physrep.2004.08.031 |s2cid=118979310 }}

{{see also|Friedmann equations}}

In standard cosmological calculations, "matter" means any constituent of the universe whose energy density scales with the inverse cube of the scale factor, i.e., {{nobr|{{math|ρ ∝ a{{sup|−3}} }}.}} This is in contrast to "radiation", which scales as the inverse fourth power of the scale factor {{nobr|{{math|ρ ∝ a{{sup|−4}} }},}} and a cosmological constant, which does not change with respect to {{mvar|a}} ({{nobr|{{math|ρ ∝ a{{sup|0}}}}}}). The different scaling factors for matter and radiation are a consequence of radiation redshift. For example, after doubling the diameter of the observable Universe via cosmic expansion, the scale, {{mvar|a}}, has doubled. The energy of the cosmic microwave background radiation has been halved (because the wavelength of each photon has doubled);{{cite news |last1=Siegel |first1=Ethan |title=Is energy conserved when photons redshift in our expanding universe? |year=2019 |work=Starts With a Bang |url=https://www.forbes.com/sites/startswithabang/2019/08/14/is-energy-conserved-when-photons-redshift-due-to-the-expanding-universe/?sh=745b3e3a3efa |access-date=5 November 2022 |language=en}} the energy of ultra-relativistic particles, such as early-era standard-model neutrinos, is similarly halved.{{efn|

However, in the modern cosmic era, this neutrino field has cooled and started to behave more like matter and less like radiation.

}} The cosmological constant, as an intrinsic property of space, has a constant energy density regardless of the volume under consideration.{{cite web |first=Daniel |last=Baumann |title=Cosmology: Part III |department=Mathematical Tripos |publisher=Cambridge University |pages=21–22 |url=http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf |access-date=24 January 2017 |archive-url=https://web.archive.org/web/20170202065045/http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf |archive-date=2 February 2017 }}

In principle, "dark matter" means all components of the universe which are not visible but still obey {{nobr|{{math|ρ ∝ a{{sup|−3}} }}.}} In practice, the term "dark matter" is often used to mean only the non-baryonic component of dark matter, i.e., excluding "missing baryons".{{cite arXiv |author=Peter |first=Annika H. G. |date=18 Jan 2012 |title=Dark Matter: A Brief Review |class=astro-ph.CO |eprint=1201.3942 }} Context will usually indicate which meaning is intended.

This section presents the observational evidence for dark matter from the smallest to the largest scales.

{{Main|Galaxy rotation curve}}

File:Comparison of rotating disc galaxies in the distant Universe and the present day.webm

The arms of spiral galaxies rotate around their galactic center. The luminous mass density of a spiral galaxy decreases as one goes from the center to the outskirts. If luminous mass were all the matter, then the galaxy can be modelled as a point mass in the centre and test masses orbiting around it, similar to the Solar System.This is a consequence of the shell theorem and the observation that spiral galaxies are spherically symmetric to a large extent (in 2D). From Kepler's Third Law, it is expected that the rotation velocities will decrease with distance from the center, similar to the Solar System. This is not observed.{{cite journal |last=Salucci |first=P. |title=The distribution of dark matter in galaxies |journal=The Astronomy and Astrophysics Review |date=2019 |volume= 27 |issue= 1 |pages= 2 |doi=10.1007/s00159-018-0113-1 |arxiv= 1811.08843 |bibcode= 2019A&ARv..27....2S}} Instead, the galaxy rotation curve remains flat or even increases as distance from the center increases.

If Kepler's laws are correct, then the obvious way to resolve this discrepancy is to conclude the mass distribution in spiral galaxies is not similar to that of the Solar System. In particular, there may be a lot of non-luminous matter (dark matter) in the outskirts of the galaxy.

{{Main|Velocity dispersion}}

Stars in bound systems must obey the virial theorem. The theorem, together with the measured velocity distribution, can be used to measure the mass distribution in a bound system, such as elliptical galaxies or globular clusters. With some exceptions, velocity dispersion estimates of elliptical galaxies{{cite journal |last1=Faber |first1=S. M. |last2=Jackson |first2=R. E. |date=1976 |title=Velocity dispersions and mass-to-light ratios for elliptical galaxies |journal=The Astrophysical Journal |volume=204 |pages=668–683 |bibcode=1976ApJ...204..668F |doi=10.1086/154215}} do not match the predicted velocity dispersion from the observed mass distribution, even assuming complicated distributions of stellar orbits.{{cite book |last1=Binny |first1=James |last2=Merrifield |first2=Michael |year=1998 |title=Galactic Astronomy |publisher=Princeton University Press |pages=712–713}}

As with galaxy rotation curves, the obvious way to resolve the discrepancy is to postulate the existence of non-luminous matter.

Galaxy clusters are particularly important for dark matter studies since their masses can be estimated in three independent ways:

• From the scatter in radial velocities of the galaxies within clusters
• From X-rays emitted by hot gas in the clusters. From the X-ray energy spectrum and flux, the gas temperature and density can be estimated, hence giving the pressure; assuming pressure and gravity balance determines the cluster's mass profile.
• Gravitational lensing (usually of more distant galaxies) can measure cluster masses without relying on observations of dynamics (e.g., velocity).

Generally, these three methods are in reasonable agreement that dark matter outweighs visible matter by approximately 5 to 1.{{cite journal |title=Cosmological Parameters from Clusters of Galaxies |journal=Annual Review of Astronomy and Astrophysics |volume=49 |issue=1 |pages=409–470 |doi=10.1146/annurev-astro-081710-102514 |arxiv=1103.4829 |bibcode=2011ARA&A..49..409A |year=2011 |last1=Allen |first1=Steven W. |last2=Evrard |first2=August E. |last3=Mantz |first3=Adam B. |s2cid=54922695 }}

{{Main|Bullet Cluster}}

The Bullet Cluster is the result of a recent collision of two galaxy clusters. It is of particular note because the location of the center of mass as measured by gravitational lensing is different from the location of the center of mass of visible matter. This is difficult for modified gravity theories, which generally predict lensing around visible matter, to explain.{{cite conference |url=http://cosis.net/abstracts/COSPAR2006/02655/COSPAR2006-A-02655.pdf |archive-url=https://web.archive.org/web/20060821074820/http://www.cosis.net/abstracts/COSPAR2006/02655/COSPAR2006-A-02655.pdf |archive-date=2006-08-21 |url-status=live |title=Dark matter and the Bullet Cluster |author1=Markevitch, M. |author2=Randall, S. |author3=Clowe, D. |author4=Gonzalez, A. |author5=Bradac, M. |name-list-style=amp |conference=36th COSPAR Scientific Assembly |date=16–23 July 2006 |location=Beijing, China}} Abstract only{{cite journal |last1=Clowe |first1=Douglas |display-authors=etal |year=2006 |title=A Direct Empirical Proof of the Existence of Dark Matter |journal=The Astrophysical Journal Letters |volume=648 |issue=2 |pages=L109–L113 |arxiv=astro-ph/0608407 |doi=10.1086/508162 |bibcode=2006ApJ...648L.109C|s2cid=2897407 }}{{cite web |url=https://arstechnica.com/science/2017/09/science-in-progress-did-the-bullet-cluster-withstand-scrutiny/ |title=Science-in-progress: Did the Bullet Cluster withstand scrutiny? |website=Ars Technica |author=Lee, Chris |date=21 September 2017}}{{cite magazine |url=https://www.forbes.com/sites/startswithabang/2017/11/09/the-bullet-cluster-proves-dark-matter-exists-but-not-for-the-reason-most-physicists-think/#3032b6081738 |title=The Bullet Cluster proves dark matter exists, but not for the reason most physicists think |magazine=Forbes |author=Siegel, Ethan |author-link=Ethan Siegel |date=9 November 2017}} Standard dark matter theory however has no issue: the hot, visible gas in each cluster would be cooled and slowed down by electromagnetic interactions, while dark matter (which does not interact electromagnetically) would not. This leads to the dark matter separating from the visible gas, producing the separate lensing peak as observed.{{cite web|url=https://chandra.harvard.edu/graphics/resources/handouts/lithos/bullet_lithos.pdf|title=Bullet Cluster: Direct Proof of Dark Matter|publisher=NASA}}

One of the consequences of general relativity is the gravitational lens. Gravitational lensing occurs when massive objects between a source of light and the observer act as a lens to bend light from this source. Lensing does not depend on the properties of the mass; it only requires there to be a mass. The more massive an object, the more lensing is observed. An example is a cluster of galaxies lying between a more distant source such as a quasar and an observer. In this case, the galaxy cluster will lens the quasar.

Strong lensing is the observed distortion of background galaxies into arcs when their light passes through such a gravitational lens. It has been observed around many distant clusters including Abell 1689.{{cite journal |last1=Taylor |first1=A. N. |display-authors=etal |date=1998 |title=Gravitational lens magnification and the mass of Abell 1689 |journal=The Astrophysical Journal |volume=501 |issue=2 |pages=539–553 |arxiv=astro-ph/9801158 |bibcode=1998ApJ...501..539T |doi=10.1086/305827 |s2cid=14446661}} By measuring the distortion geometry, the mass of the intervening cluster can be obtained. In the weak regime, lensing does not distort background galaxies into arcs, causing minute distortions instead. By examining the apparent shear deformation of the adjacent background galaxies, the mean distribution of dark matter can be characterized. The measured mass-to-light ratios correspond to dark matter densities predicted by other large-scale structure measurements.{{cite journal |last1=Refregier |first1=A. |year=2003 |title=Weak gravitational lensing by large-scale structure |journal=Annual Review of Astronomy and Astrophysics |volume=41 |issue=1 |pages=645–668 |arxiv=astro-ph/0307212 |bibcode=2003ARA&A..41..645R |doi=10.1146/annurev.astro.41.111302.102207|s2cid=34450722 }}{{cite journal |last1=Wu |first1=X. |last2=Chiueh |first2=T. |last3=Fang |first3=L. |last4=Xue |first4=Y. |date=1998 |title=A comparison of different cluster mass estimates: consistency or discrepancy? |journal=Monthly Notices of the Royal Astronomical Society |volume=301 |issue=3 |pages=861–871 |arxiv=astro-ph/9808179 |bibcode=1998MNRAS.301..861W |doi=10.1046/j.1365-8711.1998.02055.x |doi-access=free |citeseerx=10.1.1.256.8523|s2cid=1291475 }}

{{Main|Type Ia supernova|Shape of the universe}}

Type Ia supernovae can be used as standard candles to measure extragalactic distances, which can in turn be used to measure how fast the universe has expanded in the past.{{cite journal |last1=Planck Collaboration |last2=Aghanim |first2=N.|author2-link=Nabila Aghanim |last3=Akrami |first3=Y. |last4=Ashdown |first4=M. |last5=Aumont |first5=J. |last6=Baccigalupi |first6=C. |last7=Ballardini |first7=M. |last8=Banday |first8=A. J. |last9=Barreiro |first9=R. B. |last10=Bartolo |first10=N. |last11=Basak |first11=S. |title=Planck 2018 results. VI. Cosmological parameters |journal=Astronomy & Astrophysics |year=2020 |volume=641 |pages=A6 |doi=10.1051/0004-6361/201833910 |arxiv=1807.06209 |bibcode=2020A&A...641A...6P |s2cid=119335614}} Data indicates the universe is expanding at an accelerating rate, the cause of which is usually ascribed to dark energy.{{cite journal |last1=Kowalski |first1=M. |display-authors=etal |year=2008 |title=Improved Cosmological Constraints from New, Old, and Combined Supernova Data Sets |journal=The Astrophysical Journal |volume=686 |issue=2 |pages=749–778 |arxiv=0804.4142 |bibcode=2008ApJ...686..749K |doi=10.1086/589937 |s2cid=119197696 }} Since observations indicate the universe is almost flat,{{cite web|url= https://map.gsfc.nasa.gov/universe/uni_shape.html |title=Will the Universe expand forever? |publisher=NASA |date=24 January 2014 |access-date=2021-03-28}}{{cite web|url= https://www.symmetrymagazine.org/article/april-2015/our-flat-universe |title=Our flat universe |publisher=FermiLab/SLAC |date=7 April 2015 |access-date=2021-03-28}}{{cite journal |title=Unexpected connections |first=Marcus Y. |last=Yoo |journal=Engineering & Science |volume=74 |issue=1 |date=2011 |page=30}} it is expected the total energy density of everything in the universe should sum to 1 ({{nowrap|Ω_tot ≈ 1}}). The measured dark energy density is {{nowrap|Ω_Λ ≈ 0.690}}; the observed ordinary (baryonic) matter energy density is {{nowrap|Ω_b ≈ 0.0482}} and the energy density of radiation is negligible. This leaves a missing {{nowrap|Ω_dm ≈ 0.258}} which nonetheless behaves like matter (see technical definition section above){{snd}}dark matter.{{cite web |url=http://www.cosmos.esa.int/web/planck/publications |title=Planck Publications: Planck 2015 Results |publisher=European Space Agency |date=February 2015 |access-date=9 February 2015}}

Large galaxy redshift surveys may be used to make a three-dimensional map of the galaxy distribution. These maps are slightly distorted because distances are estimated from observed redshifts; the redshift contains a contribution from the galaxy's so-called peculiar velocity in addition to the dominant Hubble expansion term. On average, superclusters are expanding more slowly than the cosmic mean due to their gravity, while voids are expanding faster than average. In a redshift map, galaxies in front of a supercluster have excess radial velocities towards it and have redshifts slightly higher than their distance would imply, while galaxies behind the supercluster have redshifts slightly low for their distance. This effect causes superclusters to appear squashed in the radial direction, and likewise voids are stretched. Their angular positions are unaffected. This effect is not detectable for any one structure since the true shape is not known, but can be measured by averaging over many structures. It was predicted quantitatively by Nick Kaiser in 1987, and first decisively measured in 2001 by the 2dF Galaxy Redshift Survey.{{cite journal |last1=Peacock |first1=J. |display-authors=etal |title=A measurement of the cosmological mass density from clustering in the 2dF Galaxy Redshift Survey |journal=Nature |year=2001 |volume=410 |issue=6825 |pages=169–173 |arxiv=astro-ph/0103143 |bibcode=2001Natur.410..169P |doi=10.1038/35065528 |pmid=11242069|s2cid=1546652 }} Results are in agreement with the Lambda-CDM model.

{{Main|Lyman-alpha forest}}

In astronomical spectroscopy, the Lyman-alpha forest is the sum of the absorption lines arising from the Lyman-alpha transition of neutral hydrogen in the spectra of distant galaxies and quasars. Lyman-alpha forest observations can also constrain cosmological models.{{cite journal |last1=Viel |first1=M. |last2=Bolton |first2=J. S. |last3=Haehnelt |first3=M. G. |date=2009 |title=Cosmological and astrophysical constraints from the Lyman α forest flux probability distribution function |journal=Monthly Notices of the Royal Astronomical Society |volume=399 |issue=1 |pages=L39–L43 |arxiv=0907.2927 |bibcode=2009MNRAS.399L..39V |doi=10.1111/j.1745-3933.2009.00720.x |s2cid=12470622 |doi-access=free}} These constraints agree with those obtained from WMAP data.

{{Main|Cosmic microwave background}}

Although both dark matter and ordinary matter are matter, they do not behave in the same way. In particular, in the early universe, ordinary matter was ionized and interacted strongly with radiation via Thomson scattering. Dark matter does not interact directly with radiation, but it does affect the cosmic microwave background (CMB) by its gravitational potential (mainly on large scales) and by its effects on the density and velocity of ordinary matter. Ordinary and dark matter perturbations, therefore, evolve differently with time and leave different imprints on the CMB.

The CMB is very close to a perfect blackbody but contains very small temperature anisotropies of a few parts in 100,000. A sky map of anisotropies can be decomposed into an angular power spectrum, which is observed to contain a series of acoustic peaks at near-equal spacing but different heights. The locations of these peaks depend on cosmological parameters. Matching theory to data, therefore, constrains cosmological parameters.The details are technical. For an intermediate-level introduction, see {{cite web |author=Hu, Wayne |title=Intermediate Guide to the Acoustic Peaks and Polarization |url=http://background.uchicago.edu/~whu/intermediate/intermediate.html |year=2001}}

The CMB anisotropy was first discovered by COBE in 1992, though this had too coarse resolution to detect the acoustic peaks.

After the discovery of the first acoustic peak by the balloon-borne BOOMERanG experiment in 2000, the power spectrum was precisely observed by WMAP in 2003–2012, and even more precisely by the Planck spacecraft in 2013–2015. The results support the Lambda-CDM model.{{cite journal |last1=Hinshaw |first1=G. |display-authors=etal |year=2009 |title=Five-year Wilkinson microwave anisotropy probe (WMAP) observations: Data processing, sky maps, and basic results |journal=The Astrophysical Journal Supplement |volume=180 |issue=2 |pages=225–245 |arxiv=0803.0732 |bibcode=2009ApJS..180..225H |doi=10.1088/0067-0049/180/2/225|s2cid=3629998 }}

The observed CMB angular power spectrum provides powerful evidence in support of dark matter, as its precise structure is well fitted by the Lambda-CDM model,{{cite journal |last1=Ade |first1=P.A.R. |display-authors=etal |title=Planck 2015 results. XIII. Cosmological parameters |journal=Astron. Astrophys. |year=2016 |volume=594 |issue=13 |page=A13 |doi=10.1051/0004-6361/201525830 |bibcode=2016A&A...594A..13P |arxiv=1502.01589 |s2cid=119262962 }} but difficult to reproduce with any competing model such as modified Newtonian dynamics (MOND).{{cite journal |last1=Skordis |first1=C. |display-authors=etal |title=Large scale structure in Bekenstein's theory of relativistic modified Newtonian dynamics |journal=Phys. Rev. Lett. |year=2006 |volume=96 |issue=1 |page=011301 |doi=10.1103/PhysRevLett.96.011301 |pmid=16486433 |bibcode=2006PhRvL..96a1301S |arxiv=astro-ph/0505519|s2cid=46508316 }}

{{Main|Structure formation}}

File:Dark matter map of KiDS survey region (region G12).jpgStructure formation refers to the period after the Big Bang when density perturbations collapsed to form stars, galaxies, and clusters. Prior to structure formation, the Friedmann solutions to general relativity describe a homogeneous universe. Later, small anisotropies gradually grew and condensed the homogeneous universe into stars, galaxies and larger structures. Ordinary matter is affected by radiation, which is the dominant element of the universe at very early times. As a result, its density perturbations are washed out and unable to condense into structure.{{cite web |author=Jaffe |first=A. H. |title=Cosmology 2012: Lecture Notes |url=http://astro.imperial.ac.uk/sites/default/files/cosmology.pdf |archive-url=https://web.archive.org/web/20160717223916/http://astro.imperial.ac.uk/sites/default/files/cosmology.pdf |archive-date=July 17, 2016}} If there were only ordinary matter in the universe, there would not have been enough time for density perturbations to grow into the galaxies and clusters currently seen.

Dark matter provides a solution to this problem because it is unaffected by radiation. Therefore, its density perturbations can grow first. The resulting gravitational potential acts as an attractive potential well for ordinary matter collapsing later, speeding up the structure formation process.{{cite journal |author=Low |first=L. F. |date=12 October 2016 |title=Constraints on the composite photon theory |url=https://zenodo.org/record/896052 |journal=Modern Physics Letters A |volume=31 |issue=36 |page=1675002 |bibcode=2016MPLA...3175002L |doi=10.1142/S021773231675002X}}

{{Main|Baryon acoustic oscillations}}

Baryon acoustic oscillations (BAO) are fluctuations in the density of the visible baryonic matter (normal matter) of the universe on large scales. These are predicted to arise in the Lambda-CDM model due to acoustic oscillations in the photon–baryon fluid of the early universe and can be observed in the cosmic microwave background angular power spectrum. BAOs set up a preferred length scale for baryons. As the dark matter and baryons clumped together after recombination, the effect is much weaker in the galaxy distribution in the nearby universe, but is detectable as a subtle (~ 1%) preference for pairs of galaxies to be separated by 147 Mpc, compared to those separated by 130–160 Mpc. This feature was predicted theoretically in the 1990s and then discovered in 2005, in two large galaxy redshift surveys, the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey.

{{cite journal |last1=Percival |first1=W. J. |display-authors=etal |year=2007 |title=Measuring the Baryon Acoustic Oscillation scale using the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey |journal=Monthly Notices of the Royal Astronomical Society |volume=381 |issue=3 |pages=1053–1066 |arxiv=0705.3323 |bibcode=2007MNRAS.381.1053P |doi=10.1111/j.1365-2966.2007.12268.x |doi-access=free}} Combining the CMB observations with BAO measurements from galaxy redshift surveys provides a precise estimate of the Hubble constant and the average matter density in the Universe.{{cite journal |last1=Komatsu |first1=E. |display-authors=etal |year=2009 |title=Five-Year Wilkinson Microwave Anisotropy Probe Observations: Cosmological Interpretation |journal=The Astrophysical Journal Supplement|volume=180 |issue=2 |pages=330–376 |arxiv=0803.0547 |bibcode=2009ApJS..180..330K |doi=10.1088/0067-0049/180/2/330|s2cid=119290314 }} The results support the Lambda-CDM model.

{{Main|Cold dark matter|Warm dark matter|Hot dark matter}}

Dark matter can be divided into cold, warm, and hot categories.{{cite book |first=Joseph |last=Silk |title=The Big Bang: Third Edition |chapter-url={{google books |plainurl=y |id=XLwe1lUmz5kC |page=82}} |date=2000 |publisher=Henry Holt and Company |isbn=978-0-8050-7256-3 |chapter=IX}} These categories refer to velocity rather than an actual temperature, and indicate how far corresponding objects moved due to random motions in the early universe, before they slowed due to cosmic expansion. This distance is called the free streaming length. The categories of dark matter are set with respect to the size of the collection of mass prior to structure formation that later collapses to form a dwarf galaxy. This collection of mass is sometimes called a protogalaxy. Dark matter particles are classified as cold, warm, or hot if their free streaming length is much smaller (cold), similar to (warm), or much larger (hot) than the protogalaxy of a dwarf galaxy.{{cite book |last1=Bambi |first1=Cosimo |last2= D. Dolgov |first2=Alexandre |title=Introduction to Particle Cosmology |series=UNITEXT for Physics |year=2016 |language=English |publisher=Springer Berlin, Heidelberg | page=178 |doi=10.1007/978-3-662-48078-6 |isbn=978-3-662-48078-6}}{{cite journal |first1=N. |last1=Vittorio |author2=J. Silk |date=1984 |journal=Astrophysical Journal Letters |volume=285 |pages=L39–L43 |title=Fine-scale anisotropy of the cosmic microwave background in a universe dominated by cold dark matter |doi=10.1086/184361 |bibcode=1984ApJ...285L..39V}}{{cite journal |first1=Masayuki |last1=Umemura |author2=Satoru Ikeuchi |title=Formation of Subgalactic Objects within Two-Component Dark Matter |date=1985 |journal=Astrophysical Journal |volume=299 |pages=583–592 |doi=10.1086/163726 |bibcode=1985ApJ...299..583U}} Mixtures of the above are also possible: a theory of mixed dark matter was popular in the mid-1990s, but was rejected following the discovery of dark energy.{{Citation needed|date=April 2016}}

The significance of the free streaming length is that the universe began with some primordial density fluctuations from the Big Bang (in turn arising from quantum fluctuations at the microscale). Particles from overdense regions will naturally spread to underdense regions, but because the universe is expanding quickly, there is a time limit for them to do so. Faster particles (hot dark matter) can beat the time limit while slower particles cannot. The particles travel a free streaming length's worth of distance within the time limit; therefore this length sets a minimum scale for later structure formation. Because galaxy-size density fluctuations get washed out by free-streaming, hot dark matter implies the first objects that can form are huge supercluster-size pancakes, which then fragment into galaxies, while the reverse is true for cold dark matter.

Deep-field observations show that galaxies formed first, followed by clusters and superclusters as galaxies clump together, and therefore that most dark matter is cold. This is also the reason why neutrinos, which move at nearly the speed of light and therefore would fall under hot dark matter, cannot make up the bulk of dark matter.

File:Dark matter candidates.pdf

The identity of dark matter is unknown, but there are many hypotheses about what dark matter could consist of, as set out in the table below.

File:Fermi Observations of Dwarf Galaxies Provide New Insights on Dark Matter.ogv observations of dwarf galaxies provide new insights on dark matter.]]

{{Distinguish|Missing baryon problem}}

Dark matter can refer to any substance which interacts predominantly via gravity with visible matter (e.g., stars and planets). Hence in principle it need not be composed of a new type of fundamental particle but could, at least in part, be made up of standard baryonic matter, such as protons or neutrons. Most of the ordinary matter familiar to astronomers, including planets, brown dwarfs, red dwarfs, visible stars, white dwarfs, neutron stars, and black holes, fall into this category.{{cite journal |last1=Bertone |first1=Gianfranco |last2=Hooper |first2=Dan |title=History of dark matter |journal=Reviews of Modern Physics |date=15 October 2018 |volume=90 |issue=4 |page=045002 |doi=10.1103/RevModPhys.90.045002|arxiv=1605.04909 |bibcode=2018RvMP...90d5002B |s2cid=18596513 }}{{cite web |url=http://astronomy.swin.edu.au/cosmos/B/Baryonic+Matter |title=Baryonic Matter |website=COSMOS – The SAO Encyclopedia of Astronomy |publisher=Swinburne University of Technology|access-date=16 November 2022}} A black hole would ingest both baryonic and non-baryonic matter that comes close enough to its event horizon; afterwards, the distinction between the two is lost.{{cite web |title=Baryonic Matter |url=https://astronomy.swin.edu.au/cosmos/b/Baryonic+Matter |access-date=2023-10-03 |website=astronomy.swin.edu.au |publisher=Cosmos: The Swinburne Astronomy Online Encyclopedia |publication-place=Melbourne, Victoria, Australia: Swinburne University of Technology}}

These massive objects that are hard to detect are collectively known as MACHOs. Some scientists initially hoped that baryonic MACHOs could account for and explain all the dark matter.{{rp|286}}{{cite news |title=MACHOs may be out of the running as a dark matter candidate |url=https://astronomy.com/news/2016/08/machos-may-be-out-of-the-running-as-a-dark-matter-candidate |access-date=16 November 2022 |work=Astronomy.com |date=2016 |language=en}}

However, multiple lines of evidence suggest the majority of dark matter is not baryonic:

• Sufficient diffuse, baryonic gas or dust would be visible when backlit by stars.
• The theory of Big Bang nucleosynthesis predicts the observed abundance of the chemical elements. If there are more baryons, then there should also be more helium, lithium and heavier elements synthesized during the Big Bang.{{cite book |author=Weiss, Achim |url=http://www.einstein-online.info/spotlights/BBN |title=Big bang nucleosynthesis: Cooking up the first light elements |archive-url=https://web.archive.org/web/20130206021217/http://www.einstein-online.info/spotlights/BBN |archive-date=6 February 2013 |publisher=Einstein Online |volume=2 |year=2006 |page=1017 |access-date=1 June 2013 |url-status=dead }}{{cite book |last1=Raine |first1=D. |last2=Thomas |first2=T. |date=2001 |title=An Introduction to the Science of Cosmology |page=30 |publisher=IOP Publishing |isbn=978-0-7503-0405-4 |oclc=864166846 }} Agreement with observed abundances requires that baryonic matter makes up between 4–5% of the universe's critical density. In contrast, large-scale structure and other observations indicate that the total matter density is about 30% of the critical density.
• Astronomical searches for gravitational microlensing in the Milky Way found at most only a small fraction of the dark matter may be in dark, compact, conventional objects (MACHOs, etc.); the excluded range of object masses is from half the Earth's mass up to 30 solar masses, which covers nearly all the plausible candidates.{{cite journal |last1=Tisserand |first1=P. |last2=Le Guillou |first2=L. |last3=Afonso |first3=C. |last4=Albert |first4=J.N. |last5=Andersen |first5=J. |last6=Ansari |first6=R. |last7=Aubourg |first7=É. |last8=Bareyre |first8=P. |last9=Beaulieu |first9=J.P. |last10=Charlot |first10=X. |last11=Coutures |first11=C. |last12=Ferlet |first12=R. |last13=Fouqué |first13=P. |last14=Glicenstein |first14=J.F. |last15=Goldman |first15=B. |last16=Gould |first16=A. |last17=Graff |first17=D. |last18=Gros |first18=M. |last19=Haissinski |first19=J. |last20=Hamadache |first20=C. |last21=De Kat |first21=J. |last22=Lasserre |first22=T. |last23=Lesquoy |first23=É. |last24=Loup |first24=C. |last25=Magneville |first25=C. |last26=Marquette |first26=J.B. |last27=Maurice |first27=É. |last28=Maury |first28=A. |last29=Milsztajn |first29=A. |last30=Moniez |first30=M. |display-authors=6 |title=Limits on the Macho content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds |journal=Astronomy and Astrophysics |volume=469 |issue=2 |pages=387–404 |year=2007 |doi=10.1051/0004-6361:20066017 |url=https://www.researchgate.net/publication/41714676 |arxiv=astro-ph/0607207 |bibcode=2007A&A...469..387T|s2cid=15389106 }}{{cite journal |last1=Graff |first1=D. S. |last2=Freese |first2=K. |year=1996 |title=Analysis of a Hubble Space Telescope Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo |journal=The Astrophysical Journal |volume=456 |issue=1996 |page=L49 |arxiv=astro-ph/9507097 |bibcode=1996ApJ...456L..49G |doi=10.1086/309850 |s2cid=119417172}}{{cite journal |last1=Najita |first1=J. R. |last2=Tiede |first2=G. P. |last3=Carr |first3=J. S. |year=2000 |title=From Stars to Superplanets: The Low-Mass Initial Mass Function in the Young Cluster IC 348 |journal=The Astrophysical Journal |volume=541 |issue=2 |pages=977–1003 |arxiv=astro-ph/0005290 |bibcode=2000ApJ...541..977N |doi=10.1086/309477 |s2cid=55757804}}{{cite journal |arxiv=1106.2925 |title=The OGLE View of Microlensing towards the Magellanic Clouds. IV. OGLE-III SMC Data and Final Conclusions on MACHOs |journal=Monthly Notices of the Royal Astronomical Society |volume=416 |issue=4 |pages=2949–2961 |last1=Wyrzykowski |first1=L. |last2=Skowron |first2=J. |last3=Kozlowski |first3=S. |last4=Udalski |first4=A. |last5=Szymanski |first5=M.K. |last6=Kubiak |first6=M. |last7=Pietrzynski |first7=G. |last8=Soszynski |first8=I. |last9=Szewczyk |first9=O. |last10=Ulaczyk |first10=K. |last11=Poleski |first11=R. |last12=Tisserand |first12=P. |display-authors=6 |doi=10.1111/j.1365-2966.2011.19243.x |year=2011 |doi-access=free |bibcode=2011MNRAS.416.2949W|s2cid=118660865 }}{{cite arXiv |title=Death of stellar baryonic dark matter candidates |first1=Katherine |last1=Freese |eprint=astro-ph/0007444 |last2=Fields |first2=Brian |last3=Graff |first3=David |year=2000}}{{cite book |pages=4–6 |first1=Katherine |last1=Freese |arxiv=astro-ph/0002058 |bibcode=2000fist.conf...18F |doi=10.1007/10719504_3 |chapter=Death of Stellar Baryonic Dark Matter |series=ESO Astrophysics Symposia |last2=Fields |first2=Brian |last3=Graff |first3=David |year=2003 |title=The First Stars |isbn=978-3-540-67222-7 |citeseerx=10.1.1.256.6883|s2cid=119326375 }}
• Detailed analysis of the small irregularities (anisotropies) in the cosmic microwave background by WMAP and Planck indicate that around five-sixths of the total matter is in a form that only interacts significantly with ordinary matter or photons through gravitational effects.{{cite journal |first1=L. |last1=Canetti |first2=M. |last2=Drewes |first3=M. |last3=Shaposhnikov |title=Matter and Antimatter in the Universe |journal=New J. Phys. |year=2012 |volume=14 |issue=9 |page=095012 |doi=10.1088/1367-2630/14/9/095012 |arxiv=1204.4186 |bibcode=2012NJPh...14i5012C|s2cid=119233888 }}

There are two main candidates for non-baryonic dark matter: new particles and primordial black holes.

Unlike baryonic matter, nonbaryonic particles do not contribute to the formation of the elements in the early universe (Big Bang nucleosynthesis) and so its presence is felt only via its gravitational effects (such as weak lensing). In addition, some dark matter candidates can interact with themselves (self-interacting dark matter) or with ordinary particles (e.g. WIMPs or Weakly Interacting Massive Particles), possibly resulting in observable by-products such as gamma rays and neutrinos (indirect detection).{{cite journal |last1=Bertone |first1=G. |last2=Merritt |first2=D. |title=Dark Matter Dynamics and Indirect Detection |year=2005 |journal=Modern Physics Letters A |volume=20 |issue=14 |pages=1021–1036 |arxiv=astro-ph/0504422 |bibcode=2005MPLA...20.1021B |doi=10.1142/S0217732305017391|s2cid=119405319 }} Candidates abound (see the table above), each with their own strengths and weaknesses.

{{main|Weakly Interacting Massive Particles}}

There exists no formal definition of a Weakly Interacting Massive Particle, but broadly, it is an elementary particle which interacts via gravity and any other force (or forces) which is as weak as or weaker than the weak nuclear force, but also non-vanishing in strength. Many WIMP candidates are expected to have been produced thermally in the early Universe, similarly to the particles of the Standard Model{{cite journal | last = Garrett | first = Katherine | title = Dark matter: A primer | year = 2010 | journal = Advances in Astronomy | volume = 2011 | issue = 968283 | pages = 1–22 | doi = 10.1155/2011/968283| arxiv = 1006.2483 | bibcode = 2011AdAst2011E...8G | doi-access = free }} according to Big Bang cosmology, and usually will constitute cold dark matter. Obtaining the correct abundance of dark matter today via thermal production requires a self-annihilation cross section of $\backslash langle\; \backslash sigma\; v\; \backslash rangle$ ≃ {{val|3|e=-26|u=cm³⋅s⁻¹}}, which is roughly what is expected for a new particle in the 100 GeV/c² mass range that interacts via the electroweak force.

Because supersymmetric extensions of the Standard Model of particle physics readily predict a new particle with these properties, this apparent coincidence is known as the "WIMP miracle", and a stable supersymmetric partner has long been a prime explanation for dark matter.{{cite journal |last1=Jungman |first1=Gerard |last2=Kamionkowski |first2=Marc |last3=Griest |first3=Kim |year=1996 |title=Supersymmetric dark matter |journal=Physics Reports |volume=267 |issue=5–6 |pages=195–373 |s2cid=119067698 |arxiv=hep-ph/9506380 |bibcode=1996PhR...267..195J |doi=10.1016/0370-1573(95)00058-5}} Experimental efforts to detect WIMPs include the search for products of WIMP annihilation, including gamma rays, neutrinos and cosmic rays in nearby galaxies and galaxy clusters; direct detection experiments designed to measure the collision of WIMPs with nuclei in the laboratory, as well as attempts to directly produce WIMPs in colliders, such as the Large Hadron Collider at CERN.

In the early 2010s, results from direct-detection experiments along with the lack of evidence for supersymmetry at the Large Hadron Collider (LHC) experiment{{cite news |url=http://news.discovery.com/space/lhc-discovery-maims-supersymmetry-again-130724.htm |title=LHC discovery maims supersymmetry again |website=Discovery News}}{{cite arXiv |last=Craig |first=Nathaniel |year=2013 |title=The State of Supersymmetry after Run I of the LHC |class=hep-ph |eprint=1309.0528}} have cast doubt on the simplest WIMP hypothesis.{{cite journal |last1=Fox |first1=Patrick J. |last2=Jung |first2=Gabriel |last3=Sorensen |first3=Peter |last4=Weiner |first4=Neal |year=2014 |title=Dark matter in light of LUX |journal=Physical Review D |volume=89 |issue=10 |page=103526 |arxiv=1401.0216 |bibcode=2014PhRvD..89j3526F |doi=10.1103/PhysRevD.89.103526}}

{{main|Axion}}

Axions are hypothetical elementary particles originally theorized in 1978 independently by Frank Wilczek and Steven Weinberg as the Goldstone boson of Peccei–Quinn theory, which had been proposed in 1977 to solve the strong CP problem in quantum chromodynamics (QCD). QCD effects produce an effective periodic potential in which the axion field moves.{{cite book|last=Peccei | first=R. D. | title=Axions: Theory, Cosmology, and Experimental Searches |year=2008 |chapter=The Strong CP Problem and Axions |editor1-last=Kuster |editor1-first=Markus |editor2-last=Raffelt |editor2-first=Georg |editor3-last=Beltrán |editor3-first=Berta |series=Lecture Notes in Physics |volume=741 |pages=3–17 |arxiv=hep-ph/0607268 |doi=10.1007/978-3-540-73518-2_1 |isbn=978-3-540-73517-5|s2cid=119482294 }} Expanding the potential about one of its minima, one finds that the product of the axion mass with the axion decay constant is determined by the topological susceptibility of the QCD vacuum. An axion with mass that is much less than 60 keV/c² is long-lived and weakly interacting: a perfect dark matter candidate.

The oscillations of the axion field about the minimum of the effective potential, the so-called misalignment mechanism, generate a cosmological population of cold axions with an abundance depending on the mass of the axion.{{cite journal |last1=Preskill |first1=J. |author1-link=John Preskill |last2=Wise |first2=M. |author2-link=Mark B. Wise |last3=Wilczek |first3=F. |author3-link=Frank Wilczek |date=6 January 1983 |journal=Physics Letters B |volume=120 |issue=1–3 |pages=127–132 |title=Cosmology of the invisible axion |doi=10.1016/0370-2693(83)90637-8 |bibcode=1983PhLB..120..127P |url=http://www.theory.caltech.edu/~preskill/pubs/preskill-1983-axion.pdf |citeseerx=10.1.1.147.8685 }}{{cite journal |last1=Abbott |first1=L. |last2=Sikivie |first2=P. |year=1983 |journal=Physics Letters B |volume=120 |issue=1–3 |pages=133–136 |title=A cosmological bound on the invisible axion |bibcode=1983PhLB..120..133A |doi=10.1016/0370-2693(83)90638-X |citeseerx=10.1.1.362.5088}}{{cite journal |last1=Dine |first1=M. |last2=Fischler |first2=W. |year=1983 |journal=Physics Letters B |volume=120 |issue=1–3 |pages=137–141 |title=The not-so-harmless axion |doi=10.1016/0370-2693(83)90639-1 |bibcode=1983PhLB..120..137D}} With a mass above 5 electron-volt ({{10^|−11}} times the electron mass) axions could account for dark matter, and thus be both a dark-matter candidate and a solution to the strong CP problem. If inflation occurs at a low scale and lasts sufficiently long, the axion mass can be as low as 1 peV/c².{{cite journal |last1=di Luzio |first1=L. |last2=Nardi |first2=E. |last3=Giannotti |first3=M. |last4=Visinelli |first4=L. |date=25 July 2020 |journal=Physics Reports |volume=870 |pages=1–117 |title=The landscape of QCD axion models |bibcode=2020PhR...870....1D |doi=10.1016/j.physrep.2020.06.002 |arxiv=2003.01100 |s2cid=211678181 }}{{cite journal |last1=Graham |first1=Peter W. |last2=Scherlis |first2=Adam |title=Stochastic axion scenario |journal=Physical Review D |date=9 August 2018 |volume=98 |issue=3 |page=035017 |doi=10.1103/PhysRevD.98.035017 |arxiv=1805.07362 |bibcode=2018PhRvD..98c5017G |s2cid=119432896 }}{{cite journal |last1=Takahashi |first1=Fuminobu |last2=Yin |first2=Wen |last3=Guth |first3=Alan H. |title=The QCD Axion Window and Low Scale Inflation |journal=Physical Review D |date=31 July 2018 |volume=98 |issue=1 |pages=015042 |doi=10.1103/PhysRevD.98.015042 |arxiv=1805.08763 |bibcode=2018PhRvD..98a5042T |s2cid=54584447 }}

Because axions have extremely low mass, their de Broglie wavelength is very large, in turn meaning that quantum effects could help resolve the small-scale problems of the Lambda-CDM model. A single ultralight axion with a decay constant at the grand unified theory scale provides the correct relic density without fine-tuning.{{cite journal |last=Marsh |first=David J.E. |date=2016 |title=Axion cosmology |journal=Physics Reports |language=en |volume=643 |pages=1–79 |doi=10.1016/j.physrep.2016.06.005 |arxiv=1510.07633|bibcode=2016PhR...643....1M |s2cid=119264863 }}

Axions as a dark matter candidate have gained in popularity in recent years, because of the non-detection of WIMPS.{{cite web|url=https://physicsworld.com/a/dark-matters-secret-identity-wimps-or-axions/?form=MG0AV3|title=Dark matter's secret identity: WIMPs or axions?|publisher=Physics World|date = 25 June 2024}}

{{main|Primordial black hole}}

Primordial black holes are hypothetical black holes that formed soon after the Big Bang. In the inflationary era and early radiation-dominated universe, extremely dense pockets of subatomic matter may have been tightly packed to the point of gravitational collapse, creating primordial black holes without the supernova compression typically needed to make black holes today. Because the creation of primordial black holes would pre-date the first stars, they are not limited to the narrow mass range of stellar black holes and also not classified as baryonic dark matter.

The idea that black holes could form in the early universe was first suggested by Yakov Zeldovich and Igor Dmitriyevich Novikov in 1967, and independently by Stephen Hawking in 1971. It quickly became clear that such black holes might account for at least part of dark matter. Primordial black holes as a dark matter candidate has the major advantage that it is based on a well-understood theory (General Relativity) and objects (black holes) that are already known to exist. However, producing primordial black holes requires exotic cosmic inflation or physics beyond the standard model of particle physics,{{cite journal|last1=Villanueva-Domingo|first1=Pablo|last2=Mena|first2=Olga|last3=Palomares-Ruiz|first3=Sergio|title=A Brief Review on Primordial Black Holes as Dark Matter|journal=Frontiers in Astronomy and Space Sciences|date=2021 |volume=8 |page=87 |doi=10.3389/fspas.2021.681084|doi-access=free |arxiv=2103.12087 |bibcode=2021FrASS...8...87V }} and might also require fine-tuning.{{cite arXiv |last1= Carr|first1= Bernard J.|last2=Green|first2=Anne M.|date= June 2024|title= The History of Primordial Black Holes|eprint= 2406.05736v1|class=astro-ph.CO}} Primordial black holes can also span nearly the entire possible mass range, from atom-sized to supermassive.

The idea that primordial black holes make up dark matter gained prominence in 2015

{{cite journal

|last1=Cho |first1=Adrian

|date=9 February 2017

|title=Is dark matter made of black holes?

|journal=Science

|doi=10.1126/science.aal0721

}}

following results of gravitational wave measurements which detected the merger of intermediate-mass black holes. Black holes with about 30 solar masses are not predicted to form by either stellar collapse (typically less than 15 solar masses) or by the merger of black holes in galactic centers (millions or billions of solar masses), which suggests that the detected black holes might be primordial. A later survey of about a thousand supernovae detected no gravitational lensing events, when about eight would be expected if intermediate-mass primordial black holes above a certain mass range accounted for over 60% of dark matter.

{{cite news

|title=Black holes can't explain dark matter

|date=18 October 2018

|magazine=Astronomy

|via=astronomy.com

|url=http://astronomy.com/news/2018/10/can-black-holes-explain-dark-matter

|access-date=7 January 2019

}}

However, that study assumed that all black holes have the same or similar mass to the LIGO/Virgo mass range, which might not be the case (as suggested by subsequent James Webb Space Telescope observations).

{{cite journal

|last1=Zumalacárregui |first1=Miguel

|last2=Seljak |first2=Uroš

|title=Limits on Stellar-Mass Compact Objects as Dark Matter from Gravitational Lensing of Type Ia Supernovae

|journal=Physical Review Letters

|date=1 October 2018

|volume=121 |issue=14 |page=141101

|doi=10.1103/PhysRevLett.121.141101 |pmid=30339429

|arxiv=1712.02240 |bibcode=2018PhRvL.121n1101Z

|s2cid=53009603

|url=https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.141101

|access-date=17 August 2023

}}

The possibility that atom-sized primordial black holes account for a significant fraction of dark matter was ruled out by measurements of positron and electron fluxes outside the Sun's heliosphere by the Voyager 1 spacecraft. Tiny black holes are theorized to emit Hawking radiation. However the detected fluxes were too low and did not have the expected energy spectrum, suggesting that tiny primordial black holes are not widespread enough to account for dark matter.

{{cite news

|title=Aging Voyager 1 spacecraft undermines idea that dark matter is tiny black holes

|date=9 January 2019

|journal=Science

|via=sciencemag.org

|url=https://www.science.org/content/article/aging-voyager-1-spacecraft-undermines-idea-dark-matter-tiny-black-holes

|access-date=10 January 2019

}}

Nonetheless, research and theories proposing dense dark matter accounts for dark matter continue as of 2018, including approaches to dark matter cooling,

{{cite news

|first=Shannon |last=Hall

|date=5 February 2018

|title=There could be entire stars and planets made out of dark matter

|magazine=New Scientist

|url=https://www.newscientist.com/article/2160305-there-could-be-entire-stars-and-planets-made-out-of-dark-matter/

}}

{{cite journal

|last1=Buckley |first1=Matthew R.

|last2=Difranzo |first2=Anthony

|year=2018

|title=Collapsed dark matter structures

|journal=Physical Review Letters

|volume=120 |issue=5 |page=051102

|arxiv=1707.03829 |bibcode=2018PhRvL.120e1102B

|doi=10.1103/PhysRevLett.120.051102

|pmid=29481169 |s2cid=3757868

}}

and the question remains unsettled. In 2019, the lack of microlensing effects in the observation of Andromeda suggests that tiny black holes do not exist.

{{cite journal

|last=Niikura |first=Hiroko

|date=1 April 2019

|title=Microlensing constraints on primordial black holes with Subaru/HSC Andromeda observations

|journal=Nature Astronomy

|volume=3 |issue=6 |pages=524–534

|doi=10.1038/s41550-019-0723-1 |s2cid=118986293

|bibcode=2019NatAs...3..524N |arxiv=1701.02151

}}

Nonetheless, there still exists a largely unconstrained mass range smaller than that which can be limited by optical microlensing observations, where primordial black holes may account for all dark matter.

{{cite journal

|last1=Katz |first1=Andrey |last2=Kopp |first2=Joachim

|last3=Sibiryakov |first3=Sergey |last4=Xue |first4=Wei

|date=5 December 2018

|title=Femtolensing by dark matter revisited

|journal=Journal of Cosmology and Astroparticle Physics

|volume=2018 |issue=12 |page=005

|doi=10.1088/1475-7516/2018/12/005 |issn=1475-7516

|bibcode=2018JCAP...12..005K |arxiv=1807.11495

|s2cid=119215426

|url=http://stacks.iop.org/1475-7516/2018/i=12/a=005?key=crossref.dcb1e788286d8d75d0c7e7f35fe588eb

}}

{{cite journal

|last1=Montero-Camacho |first1=Paulo |last2=Fang |first2=Xiao

|last3=Vasquez |first3=Gabriel |last4=Silva |first4=Makana

|last5=Hirata |first5=Christopher M.

|date=23 August 2019

|title=Revisiting constraints on asteroid-mass primordial black holes as dark matter candidates

|journal=Journal of Cosmology and Astroparticle Physics

|volume=2019 |issue=8 |page=031

|doi=10.1088/1475-7516/2019/08/031

|issn=1475-7516 |bibcode=2019JCAP...08..031M

|arxiv=1906.05950 |s2cid=189897766

}}

{{Further|Alternatives to general relativity}}

The last major possibility is that general relativity, the theory underpinning modern cosmology, is incorrect. General relativity is well-tested on Solar System scales, but its validity on galactic or cosmological scales has not been well proven.{{cite book | author = Peebles, P. J. E.| date= December 2004 | arxiv= astro-ph/0410284|bibcode = 2005grg..conf..106P |doi = 10.1142/9789812701688_0010 | isbn = 978-981-256-424-5 | pages = 106–117 | chapter= Probing General Relativity on the Scales of Cosmology| title= General Relativity and Gravitation| s2cid= 1700265}} A suitable modification to general relativity can conceivably eliminate the need for dark matter. The best-known theories of this class are MOND and its relativistic generalization tensor–vector–scalar gravity (TeVeS),For a review, see: {{cite journal |title=The failures of the Standard Model of Cosmology require a new paradigm |author1=Kroupa, Pavel |display-authors=etal |journal=International Journal of Modern Physics D |date=December 2012 |volume=21 |issue=4 |page=1230003 |doi=10.1142/S0218271812300030 |arxiv=1301.3907 |bibcode=2012IJMPD..2130003K|s2cid=118461811 }} f(R) gravity,For a review, see: {{cite journal |title=The dark matter problem from f(R) gravity viewpoint |author=Salvatore Capozziello |author2=Mariafelicia De Laurentis |journal=Annalen der Physik |date=October 2012 |volume=524 |issue=9–10 |page=545 |doi=10.1002/andp.201200109 |bibcode=2012AnP...524..545C |doi-access=free }} negative mass, dark fluid,{{cite web|url= https://www.ox.ac.uk/news/2018-12-05-bringing-balance-universe |title=Bringing balance to the Universe |date=5 December 2018 |publisher=University of Oxford}}{{cite web|url= https://phys.org/news/2018-12-universe-theory-percent-cosmos.html |title=Bringing balance to the universe: New theory could explain missing 95 percent of the cosmos |publisher=Phys.Org}}{{cite journal |last=Farnes |first=J. S. |year=2018 |title=A Unifying Theory of Dark Energy and Dark Matter: Negative Masses and Matter Creation within a Modified ΛCDM Framework |journal=Astronomy & Astrophysics |volume=620 |page=A92 |arxiv=1712.07962 |bibcode=2018A&A...620A..92F |doi=10.1051/0004-6361/201832898 |s2cid=53600834}} and entropic gravity.{{cite news |title=New theory of gravity might explain dark matter |url=https://phys.org/news/2016-11-theory-gravity-dark.html |website=phys.org |date=November 2016}} Alternative theories abound.{{cite journal |title=Alternatives to dark matter and dark energy |first=Phillip D. |last=Mannheim |journal=Progress in Particle and Nuclear Physics |volume=56 |issue=2 |pages=340–445 |doi=10.1016/j.ppnp.2005.08.001 |arxiv=astro-ph/0505266 |date=April 2006 |bibcode=2006PrPNP..56..340M |s2cid=14024934 }}{{cite journal |title=Beyond the Cosmological Standard Model |first1=Austin |last1=Joyce |display-authors=etal |journal=Physics Reports |date=March 2015 |volume=568 |pages=1–98 |doi=10.1016/j.physrep.2014.12.002 |arxiv=1407.0059 |bibcode=2015PhR...568....1J|s2cid=119187526 }}

A problem with modifying gravity is that observational evidence for dark matter – let alone general relativity – comes from so many independent approaches (see the "observational evidence" section above). Explaining any individual observation is possible but explaining all of them in the absence of dark matter is very difficult. Nonetheless, there have been some scattered successes for alternative hypotheses, such as a 2016 test of gravitational lensing in entropic gravity{{cite news |url=http://phys.org/news/2016-12-verlinde-theory-gravity.html |title=Verlinde's new theory of gravity passes first test |date=16 December 2016}}{{cite journal |title=First test of Verlinde's theory of Emergent Gravity using Weak Gravitational Lensing measurements |first1=Margot M. |last1=Brouwer |display-authors=etal |journal=Monthly Notices of the Royal Astronomical Society |volume=466 |issue=3 |date=April 2017 |doi=10.1093/mnras/stw3192 |arxiv=1612.03034 |pages=2547–2559 |doi-access=free |bibcode=2017MNRAS.466.2547B|s2cid=18916375 }}{{cite web |url=https://www.newscientist.com/article/2116446-first-test-of-rival-to-einsteins-gravity-kills-off-dark-matter/ |title=First test of rival to Einstein's gravity kills off dark matter |date=15 December 2016 |access-date=20 February 2017}} and a 2020 measurement of a unique MOND effect.{{cite web|url=https://www.sciencedaily.com/releases/2020/12/201216155158.htm|title=Unique prediction of 'modified gravity' challenges dark matter|publisher=ScienceDaily|date=16 December 2020|access-date=14 January 2021}}{{cite journal|title=Testing the Strong Equivalence Principle: Detection of the External Field Effect in Rotationally Supported Galaxies|first1=Kyu-Hyun|last1=Chae|display-authors=et al|journal=Astrophysical Journal|volume=904|date=20 November 2020|issue=1|page=51|doi=10.3847/1538-4357/abbb96|arxiv=2009.11525|bibcode=2020ApJ...904...51C|s2cid=221879077 |doi-access=free }}

The prevailing opinion among most astrophysicists is that while modifications to general relativity can conceivably explain part of the observational evidence, there is probably enough data to conclude there must be some form of dark matter present in the universe.{{cite web |author=Carroll |first=Sean |author-link=Sean M. Carroll |date=9 May 2012 |title=Dark matter vs. modified gravity: A trialogue |url=http://www.preposterousuniverse.com/blog/2012/05/09/dark-matter-vs-modified-gravity-a-trialogue/ |access-date=14 February 2017}}

If dark matter is composed of weakly interacting particles, then an obvious question is whether it can form objects equivalent to planets, stars, or black holes. Historically, the answer has been it cannot,{{efn|

"One widely held belief about dark matter is it cannot cool off by radiating energy. If it could, then it might bunch together and create compact objects in the same way baryonic matter forms planets, stars, and galaxies. Observations so far suggest dark matter doesn't do that – it resides only in diffuse halos ... As a result, it is extremely unlikely there are very dense objects like stars made out of entirely (or even mostly) {{nobr|dark matter." — Buckley & Difranzo (2018)}}

}}

{{cite journal

|last1=Buckley |first1=Matthew R.

|last2=Difranzo |first2=Anthony

|date=1 February 2018

|title=Synopsis: A way to cool dark matter

|journal=Physical Review Letters

|volume=120 |issue=5 |page=051102

|doi=10.1103/PhysRevLett.120.051102

|bibcode=2018PhRvL.120e1102B |pmid=29481169

|arxiv=1707.03829 |s2cid=3757868

|url=https://physics.aps.org/articles/v11/s15

|archive-url=https://archive.today/20201026224145/https://physics.aps.org/articles/v11/s15

|archive-date=26 October 2020

}}

{{cite web

|title=Are there any dark stars or dark galaxies made of dark matter?

|department=Ask an Astronomer

|website=curious.astro.cornell.edu

|publisher=Cornell University

|url=http://curious.astro.cornell.edu/about-us/95-the-universe/galaxies/general-questions/508-are-there-any-dark-stars-or-dark-galaxies-made-of-dark-matter-advanced

|archive-url=https://web.archive.org/web/20150302105015/http://curious.astro.cornell.edu/about-us/95-the-universe/galaxies/general-questions/508-are-there-any-dark-stars-or-dark-galaxies-made-of-dark-matter-advanced

|archive-date=2 March 2015

}}

{{cite magazine

|author-link=Ethan Siegel |author=Siegel, Ethan

|date=28 October 2016

|title=Why doesn't dark matter form black holes?

|magazine=Forbes

|url=https://www.forbes.com/sites/startswithabang/2016/10/28/why-doesnt-dark-matter-form-black-holes/#4e5014943de1

}}

because of two factors:

; It lacks an efficient means to lose energy

: Ordinary matter forms dense objects because it has numerous ways to lose energy. Losing energy would be essential for object formation, because a particle that gains energy during compaction or falling "inward" under gravity, and cannot lose it any other way, will heat up and increase velocity and momentum. Dark matter appears to lack a means to lose energy, simply because it is not capable of interacting strongly in other ways except through gravity. The virial theorem suggests that such a particle would not stay bound to the gradually forming object – as the object began to form and compact, the dark matter particles within it would speed up and tend to escape.

; It lacks a diversity of interactions needed to form structures

: Ordinary matter interacts in many different ways, which allows the matter to form more complex structures. For example, stars form through gravity, but the particles within them interact and can emit energy in the form of neutrinos and electromagnetic radiation through fusion when they become energetic enough. Protons and neutrons can bind via the strong interaction and then form atoms with electrons largely through electromagnetic interaction. There is no evidence that dark matter is capable of such a wide variety of interactions, since it seems to only interact through gravity (and possibly through some means no stronger than the weak interaction, although until dark matter is better understood, this is only speculation).

If dark matter is made up of subatomic particles, then millions, possibly billions, of such particles must pass through every square centimeter of the Earth each second.{{cite journal |last=Gaitskell |first=Richard J. |s2cid=11316578 |title=Direct Detection of Dark Matter |journal=Annual Review of Nuclear and Particle Science |volume=54 |pages=315–359 |bibcode=2004ARNPS..54..315G |date=2004 |doi=10.1146/annurev.nucl.54.070103.181244|doi-access=free}}{{cite web |title=Neutralino Dark Matter |url=http://www.picassoexperiment.ca/dm_neutralino.php |access-date=26 December 2011}}

{{cite web |title=WIMPs and MACHOs |url=http://www.astro.caltech.edu/~george/ay20/eaa-wimps-machos.pdf |archive-url=https://web.archive.org/web/20060923123531/http://www.astro.caltech.edu/~george/ay20/eaa-wimps-machos.pdf |archive-date=2006-09-23 |url-status=live |last=Griest |first=Kim |access-date=26 December 2011}} Many experiments aim to test this hypothesis. Although WIMPs have been the main search candidates, axions have drawn renewed attention, with the Axion Dark Matter Experiment (ADMX) searches for axions and many more planned in the future.{{cite journal |last1=Chadha-Day |first1=Francesca |last2=Ellis |first2=John |last3=Marsh |first3=David J. E. |date=23 February 2022 |title=Axion dark matter: What is it and why now? |journal=Science Advances |volume=8 |issue=8 |pages=eabj3618 |arxiv=2105.01406 |bibcode=2022SciA....8J3618C |doi=10.1126/sciadv.abj3618 |pmc=8865781 |pmid=35196098}} Another candidate is heavy hidden sector particles which only interact with ordinary matter via gravity.

These experiments can be divided into two classes: direct detection experiments, which search for the scattering of dark matter particles off atomic nuclei within a detector; and indirect detection, which look for the products of dark matter particle annihilations or decays.

{{Further|Weakly interacting massive particle#Direct detection}}

{{Main|Direct detection of dark matter}}

Direct detection experiments aim to observe low-energy recoils of nuclei (typically a few keV) induced by interactions with particles of dark matter, which (in theory) are passing through the Earth. After such a recoil, the nucleus will emit energy in the form of scintillation light or phonons as they pass through sensitive detection apparatus. To do so effectively, it is crucial to maintain an extremely low background, which is the reason why such experiments typically operate deep underground, where interference from cosmic rays is minimized. Examples of underground laboratories with direct detection experiments include the Stawell mine, the Soudan mine, the SNOLAB underground laboratory at Sudbury, the Gran Sasso National Laboratory, the Canfranc Underground Laboratory, the Boulby Underground Laboratory, the Deep Underground Science and Engineering Laboratory and the China Jinping Underground Laboratory.

These experiments mostly use either cryogenic or noble liquid detector technologies. Cryogenic detectors operating at temperatures below 100 mK, detect the heat produced when a particle hits an atom in a crystal absorber such as germanium. Noble liquid detectors detect scintillation produced by a particle collision in liquid xenon or argon. Cryogenic detector experiments include such projects as CDMS, CRESST, EDELWEISS, and EURECA, while noble liquid experiments include LZ, XENON, DEAP, ArDM, WARP, DarkSide, PandaX, and LUX, the Large Underground Xenon experiment. Both of these techniques focus strongly on their ability to distinguish background particles (which predominantly scatter off electrons) from dark matter particles (that scatter off nuclei). Other experiments include SIMPLE and PICASSO, which use alternative methods in their attempts to detect dark matter.

Currently there has been no well-established claim of dark matter detection from a direct detection experiment, leading instead to strong upper limits on the mass and interaction cross section with nucleons of such dark matter particles.{{cite journal |last1=Drees |first1=M. |last2=Gerbier |first2=G. |title=Dark Matter |journal=Chin. Phys. C |date=2015 |volume=38 |page=090001 |url=http://pdg.lbl.gov/2015/reviews/rpp2015-rev-dark-matter.pdf |archive-url=https://web.archive.org/web/20160722093442/http://pdg.lbl.gov/2015/reviews/rpp2015-rev-dark-matter.pdf |archive-date=2016-07-22 |url-status=live}} The DAMA/NaI and more recent DAMA/LIBRA experimental collaborations have detected an annual modulation in the rate of events in their detectors,{{cite journal |last1=Bernabei |first1=R. |last2=Belli |first2=P. |last3=Cappella |first3=F. |last4=Cerulli |first4=R. |last5=Dai |first5=C. J. |last6=d'Angelo |first6=A. |last7=He |first7=H. L. |last8=Incicchitti |first8=A. |last9=Kuang |first9=H. H. |last10=Ma |first10=J. M. |last11=Montecchia |first11=F. |last12=Nozzoli |first12=F. |last13=Prosperi |first13=D. |last14=Sheng |first14=X. D. |last15=Ye |first15=Z. P. |display-authors=6 |year=2008 |title=First results from DAMA/LIBRA and the combined results with DAMA/NaI |journal=Eur. Phys. J. C |volume=56 |issue=3 |pages=333–355 |arxiv=0804.2741 |bibcode=2008EPJC...56..333B |doi=10.1140/epjc/s10052-008-0662-y |s2cid=14354488}}{{cite journal |doi=10.1103/PhysRevD.33.3495 |pmid=9956575 |author1=Drukier, A. |author2=Freese, K. |author3=Spergel, D. |title=Detecting Cold Dark Matter Candidates |journal=Physical Review D |volume=33 |issue=12 |pages=3495–3508 |date=1986 |bibcode=1986PhRvD..33.3495D}} which they claim is due to dark matter. This results from the expectation that as the Earth orbits the Sun, the velocity of the detector relative to the dark matter halo will vary by a small amount. This claim is so far unconfirmed and in contradiction with negative results from other experiments such as LUX, SuperCDMS{{cite journal |last1=Davis |first1=Jonathan H. |title=The past and future of light dark matter direct detection |journal=Int. J. Mod. Phys. A |year=2015 |volume=30 |issue=15 |page=1530038 |doi=10.1142/S0217751X15300380 |arxiv=1506.03924 |bibcode=2015IJMPA..3030038D |s2cid=119269304 }} and XENON100.{{cite journal |last1=Aprile |first1=E. |title=Search for electronic recoil event rate modulation with 4 years of XENON100 data |journal=Phys. Rev. Lett. |year=2017 |volume=118 |issue=10 |page=101101 |doi=10.1103/PhysRevLett.118.101101 |pmid=28339273 |arxiv=1701.00769|bibcode=2017PhRvL.118j1101A |s2cid=206287497 }}

A special case of direct detection experiments covers those with directional sensitivity. This is a search strategy based on the motion of the Solar System around the Galactic Center.{{cite news |last=Stonebraker |first=Alan |title=Synopsis: Dark-Matter Wind Sways through the Seasons |newspaper=Physics – Synopses |publisher=American Physical Society |date=3 January 2014 |doi=10.1103/PhysRevLett.112.011301 }}{{cite journal |last1=Lee |first1=Samuel K. |first2=Mariangela |last2=Lisanti |first3=Annika H.G.|last3=Peter |first4=Benjamin R.|last4=Safdi |doi=10.1103/PhysRevLett.112.011301 |arxiv=1308.1953 |title=Effect of Gravitational Focusing on Annual Modulation in Dark-Matter Direct-Detection Experiments |date=3 January 2014 |journal=Phys. Rev. Lett. |volume=112 |issue=1 |page=011301 [5 pages] |bibcode=2014PhRvL.112a1301L |pmid=24483881|s2cid=34109648 }}{{cite news |last=The Dark Matter Group |title=An Introduction to Dark Matter |newspaper=Dark Matter Research |location=Sheffield |publisher=University of Sheffield |url=http://www.hep.shef.ac.uk/research/dm/intro.php |access-date=7 January 2014 |archive-date=29 July 2020 |archive-url=https://web.archive.org/web/20200729020742/http://www.hep.shef.ac.uk/research/dm/intro.php }}{{cite news |quote=Scientists at Kavli MIT are working on ... a tool to track the movement of dark matter. |title=Blowing in the Wind |newspaper=Kavli News |location=Sheffield |publisher=Kavli Foundation |url=http://www.kavlifoundation.org/science-spotlights/blowing-wind |access-date=7 January 2014 |archive-date=7 October 2020 |archive-url=https://web.archive.org/web/20201007192326/http://www.kavlifoundation.org/science-spotlights/blowing-wind |url-status=dead }} A low-pressure time projection chamber makes it possible to access information on recoiling tracks and constrain WIMP-nucleus kinematics. WIMPs coming from the direction in which the Sun travels (approximately towards Cygnus) may then be separated from background, which should be isotropic. Directional dark matter experiments include DMTPC, DRIFT, Newage and MIMAC.

{{main|Indirect detection of dark matter}}

File:Collage of six cluster collisions with dark matter maps.jpg

File:Turning Black Holes into Dark Matter Labs.webm of dark matter annihilation around supermassive black holes. (Duration 0:03:13, also see file description.)]]

Indirect detection experiments search for the products of the self-annihilation or decay of dark matter particles in outer space. For example, in regions of high dark matter density (e.g., the centre of the Milky Way) two dark matter particles could annihilate to produce gamma rays or Standard Model particle–antiparticle pairs.{{cite book |first=Gianfranco |last=Bertone |title=Particle Dark Matter: Observations, Models and Searches |chapter-url=https://books.google.com/books?id=JkUgAwAAQBAJ&pg=PA83 |year=2010 |publisher=Cambridge University Press |pages=83–104 |chapter=Dark Matter at the Centers of Galaxies |arxiv=1001.3706 |isbn=978-0-521-76368-4 |bibcode=2010arXiv1001.3706M}} Alternatively, if a dark matter particle is unstable, it could decay into Standard Model (or other) particles. These processes could be detected indirectly through an excess of gamma rays, antiprotons or positrons emanating from high density regions in the Milky Way and other galaxies.{{cite journal |last1=Ellis |first1=J. |last2=Flores |first2=R. A. |last3=Freese |first3=K. |last4=Ritz |first4=S. |last5=Seckel |first5=D. |last6=Silk |first6=J. |year=1988 |title=Cosmic ray constraints on the annihilations of relic particles in the galactic halo |url=https://cds.cern.ch/record/190709/files/198809398.pdf |url-status=live |journal=Physics Letters B |volume=214 |issue=3 |pages=403–412 |bibcode=1988PhLB..214..403E |doi=10.1016/0370-2693(88)91385-8 |archive-url=https://web.archive.org/web/20180728133226/https://cds.cern.ch/record/190709/files/198809398.pdf |archive-date=2018-07-28}} A major difficulty inherent in such searches is that various astrophysical sources can mimic the signal expected from dark matter, and so multiple signals are likely required for a conclusive discovery.

A few of the dark matter particles passing through the Sun or Earth may scatter off atoms and lose energy. Thus dark matter may accumulate at the center of these bodies, increasing the chance of collision/annihilation. This could produce a distinctive signal in the form of high-energy neutrinos.{{cite journal |doi=10.1016/0370-2693(86)90349-7 |author=Freese, K. |title=Can Scalar Neutrinos or Massive Dirac Neutrinos be the Missing Mass? |journal=Physics Letters B |volume=167 |issue=3 |pages=295–300 |date=1986 |bibcode=1986PhLB..167..295F}} Such a signal would be strong indirect proof of WIMP dark matter. High-energy neutrino telescopes such as AMANDA, IceCube and ANTARES are searching for this signal.

{{cite book

|first=Lisa |last=Randall

|year=2015

|title=Dark Matter and the Dinosaurs: The astounding interconnectedness of the Universe

|publisher=Ecco / HarperCollins Publishers

|location=New York, NY

|isbn=978-0-06-232847-2

}}{{rp|298}} The detection by LIGO in September 2015 of gravitational waves opens the possibility of observing dark matter in a new way, particularly if it is in the form of primordial black holes.{{cite magazine |url=https://www.newscientist.com/article/2077800-what-will-gravitational-waves-tell-us-about-the-universe |title=Surfing gravity's waves |magazine=New Scientist |first=Joshua |last=Sokol |display-authors=etal |issue=3061 |date=20 February 2016}}{{cite web |publisher=Johns Hopkins University |title=Did gravitational wave detector find dark matter? |date=15 June 2016 |url=http://releases.jhu.edu/2016/06/15/did-gravitational-wave-detector-find-dark-matter/ |access-date=20 June 2015 |quote=While their existence has not been established with certainty, primordial black holes have in the past been suggested as a possible solution to the dark matter mystery. Because there is so little evidence of them, though, the primordial black hole–dark matter hypothesis has not gained a large following among scientists. The LIGO findings, however, raise the prospect anew, especially as the objects detected in that experiment conform to the mass predicted for dark matter. Predictions made by scientists in the past held conditions at the birth of the universe would produce many of these primordial black holes distributed approximately evenly in the universe, clustering in halos around galaxies. All this would make them good candidates for dark matter.}}{{cite journal |last1=Bird |first1=Simeon |last2=Cholis |first2=Illian |year=2016 |title=Did LIGO detect dark matter? |journal=Physical Review Letters |volume=116 |issue=20 |page=201301 |doi=10.1103/PhysRevLett.116.201301 |pmid=27258861 |bibcode=2016PhRvL.116t1301B |arxiv=1603.00464|s2cid=23710177 }}

Many experimental searches have been undertaken to look for such emission from dark matter annihilation or decay, examples of which follow.

The Energetic Gamma Ray Experiment Telescope observed more gamma rays in 2008 than expected from the Milky Way, but scientists concluded this was most likely due to incorrect estimation of the telescope's sensitivity.{{cite journal |last1=Stecker |first1=F. W. |last2=Hunter |first2=S. |last3=Kniffen |first3=D. |year=2008 |title=The likely cause of the EGRET GeV anomaly and its implications |journal=Astroparticle Physics |volume=29 |issue=1 |pages=25–29 |arxiv=0705.4311 |bibcode=2008APh....29...25S |doi=10.1016/j.astropartphys.2007.11.002 |s2cid=15107441}}

The Fermi Gamma-ray Space Telescope is searching for similar gamma rays.{{cite journal |title=The large area telescope on the Fermi Gamma-ray Space Telescope Mission |journal=Astrophysical Journal |volume=697 |issue=2 |year=2009 |pages=1071–1102 |doi=10.1088/0004-637X/697/2/1071 |arxiv=0902.1089 |first1=W.B. |last1=Atwood |last2=Abdo |first2=A.A. |last3=Ackermann |first3=M. |last4=Althouse |first4=W. |last5=Anderson |first5=B. |last6=Axelsson |first6=M. |last7=Baldini |first7=L. |last8=Ballet |first8=J. |last9=Band |first9=D.L. |display-authors=6 |bibcode=2009ApJ...697.1071A|s2cid=26361978 }} In 2009, an as yet unexplained surplus of gamma rays from the Milky Way's galactic center was found in Fermi data. This Galactic Center GeV excess might be due to dark matter annihilation or to a population of pulsars.{{cite web |date=2019-11-12 |title=Physicists revive hunt for dark matter in the heart of the Milky Way |url=https://www.science.org/content/article/physicists-revive-hunt-dark-matter-heart-milky-way |access-date=2023-05-09 |website=www.science.org |language=en}} In April 2012, an analysis of previously available data from Fermi's Large Area Telescope instrument produced statistical evidence of a 130 GeV signal in the gamma radiation coming from the center of the Milky Way.{{cite journal |doi=10.1088/1475-7516/2012/08/007 |last=Weniger |first=Christoph |title=A tentative gamma-ray line from dark matter annihilation at the Fermi Large Area Telescope |journal=Journal of Cosmology and Astroparticle Physics |issue=8 |date=2012 |arxiv=1204.2797 |volume=2012 |page=7 |bibcode=2012JCAP...08..007W|s2cid=119229841 }} WIMP annihilation was seen as the most probable explanation.{{cite web |url=http://physicsworld.com/cws/article/news/2012/apr/24/gamma-rays-hint-at-dark-matter |title=Gamma rays hint at dark matter |last1=Cartlidge |first1=Edwin |date=24 April 2012 |publisher=Institute of Physics |access-date=23 April 2013}}

At higher energies, ground-based gamma-ray telescopes have set limits on the annihilation of dark matter in dwarf spheroidal galaxies{{Cite journal |last1=Albert |first1=J. |last2=Aliu |first2=E. |last3=Anderhub |first3=H. |last4=Antoranz |first4=P. |last5=Backes |first5=M. |last6=Baixeras |first6=C. |last7=Barrio |first7=J.A. |last8=Bartko |first8=H. |last9=Bastieri |first9=D. |last10=Becker |first10=J.K. |last11=Bednarek |first11=W. |last12=Berger |first12=K. |last13=Bigongiari |first13=C. |last14=Biland |first14=A. |last15=Bock |first15=R.K. |last16=Bordas |first16=P. |last17=Bosch-Ramon |first17=V. |last18=Bretz |first18=T. |last19=Britvitch |first19=I. |last20=Camara |first20=M. |last21=Carmona |first21=E. |last22=Chilingarian |first22=A. |last23=Commichau |first23=S. |last24=Contreras |first24=J.L. |last25=Cortina |first25=J. |last26=Costado |first26=M.T. |last27=Curtef |first27=V. |last28=Danielyan |first28=V. |last29=Dazzi |first29=F. |last30=De Angelis |first30=A. |display-authors=6 |title=Upper Limit for γ-Ray Emission above 140 GeV from the Dwarf Spheroidal Galaxy Draco |doi=10.1086/529135 |journal=The Astrophysical Journal |volume=679 |issue=1 |pages=428–431 |year=2008 |arxiv=0711.2574 |bibcode=2008ApJ...679..428A|s2cid=15324383 }} and in clusters of galaxies.{{cite journal |last1=Aleksić |first1=J. |last2=Antonelli |first2=L.A. |last3=Antoranz |first3=P. |last4=Backes |first4=M. |last5=Baixeras |first5=C. |last6=Balestra |first6=S. |last7=Barrio |first7=J.A. |last8=Bastieri |first8=D. |last9=González |first9=J.B. |last10=Bednarek |first10=W. |last11=Berdyugin |first11=A. |last12=Berger |first12=K. |last13=Bernardini |first13=E. |last14=Biland |first14=A. |last15=Bock |first15=R.K. |last16=Bonnoli |first16=G. |last17=Bordas |first17=P. |last18=Tridon |first18=D.B. |last19=Bosch-Ramon |first19=V. |last20=Bose |first20=D. |last21=Braun |first21=I. |last22=Bretz |first22=T. |last23=Britzger |first23=D. |last24=Camara |first24=M. |last25=Carmona |first25=E. |last26=Carosi |first26=A. |last27=Colin |first27=P. |last28=Commichau |first28=S. |last29=Contreras |first29=J.L. |last30=Cortina |first30=J. |display-authors=6 |title=Magic Gamma-Ray Telescope observation of the Perseus Cluster of galaxies: Implications for cosmic rays, dark matter, and NGC 1275 |doi=10.1088/0004-637X/710/1/634 |journal=The Astrophysical Journal |volume=710 |issue=1 |pages=634–647 |year=2010 |arxiv=0909.3267 |bibcode=2010ApJ...710..634A|s2cid=53120203 }}

The PAMELA experiment (launched in 2006) detected excess positrons. They could be from dark matter annihilation or from pulsars. No excess antiprotons were observed.{{cite journal |last1=Adriani |first1=O. |last2=Barbarino |first2=G.C. |last3=Bazilevskaya |first3=G.A. |last4=Bellotti |first4=R. |last5=Boezio |first5=M. |last6=Bogomolov |first6=E.A. |last7=Bonechi |first7=L. |last8=Bongi |first8=M. |last9=Bonvicini |first9=V. |last10=Bottai |doi=10.1038/nature07942 |first10=S. |last11=Bruno |first11=A. |last12=Cafagna |first12=F. |last13=Campana |first13=D. |last14=Carlson |first14=P. |last15=Casolino |first15=M. |last16=Castellini |first16=G. |last17=De Pascale |first17=M.P. |last18=De Rosa |first18=G. |last19=De Simone |first19=N. |last20=Di Felice |first20=V. |last21=Galper |first21=A.M. |last22=Grishantseva |first22=L. |last23=Hofverberg |first23=P. |last24=Koldashov |first24=S.V. |last25=Krutkov |first25=S.Y. |last26=Kvashnin |first26=A.N. |last27=Leonov |first27=A. |last28=Malvezzi |first28=V. |last29=Marcelli |first29=L. |last30=Menn |first30=W. |display-authors=6 |title=An anomalous positron abundance in cosmic rays with energies 1.5–100 GeV |journal=Nature |volume=458 |issue=7238 |pages=607–609 |year=2009 |pmid=19340076 |arxiv=0810.4995 |bibcode=2009Natur.458..607A|s2cid=11675154 }}

In 2013, results from the Alpha Magnetic Spectrometer on the International Space Station indicated excess high-energy cosmic rays which could be due to dark matter annihilation.{{cite journal |title=First Result from the Alpha Magnetic Spectrometer on the International Space Station: Precision Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–350 GeV |date=3 April 2013 |journal=Physical Review Letters |author=Aguilar, M. |collaboration=AMS Collaboration |doi=10.1103/PhysRevLett.110.141102 |bibcode=2013PhRvL.110n1102A |display-authors=etal |volume=110 |issue=14 |pmid=25166975 |page=141102|doi-access=free |hdl=1721.1/81241 |hdl-access=free }}{{cite web |title=First Result from the Alpha Magnetic Spectrometer Experiment |url=http://www.ams02.org/2013/04/first-results-from-the-alpha-magnetic-spectrometer-ams-experiment/ |date=3 April 2013 |author=AMS Collaboration |access-date=3 April 2013 |archive-url=https://web.archive.org/web/20130408185229/http://www.ams02.org/2013/04/first-results-from-the-alpha-magnetic-spectrometer-ams-experiment/ |archive-date=8 April 2013 }}{{cite news |last1=Heilprin |first1=John |last2=Borenstein |first2=Seth |title=Scientists find hint of dark matter from cosmos |url=http://apnews.excite.com/article/20130403/DA5E6JAG3.html |date=3 April 2013 |agency=Associated Press |access-date=3 April 2013}}{{cite news |last=Amos |first=Jonathan |title=Alpha Magnetic Spectrometer zeroes in on dark matter |url=https://www.bbc.co.uk/news/science-environment-22016504 |date=3 April 2013 |work=BBC |access-date=3 April 2013}}{{cite web |last1=Perrotto |first1=Trent J. |last2=Byerly |first2=Josh |title=NASA TV Briefing Discusses Alpha Magnetic Spectrometer Results |url=http://www.nasa.gov/home/hqnews/2013/apr/HQ_M13-054_AMS_Findings_Briefing.html |date=2 April 2013 |website=NASA |access-date=3 April 2013}}{{cite news |last=Overbye |first=Dennis |title=New Clues to the Mystery of Dark Matter |url=https://www.nytimes.com/2013/04/04/science/space/new-clues-to-the-mystery-of-dark-matter.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2013/04/04/science/space/new-clues-to-the-mystery-of-dark-matter.html |archive-date=2022-01-01 |url-access=limited |date=3 April 2013 |work=The New York Times |access-date=3 April 2013}}{{cbignore}}

An alternative approach to the detection of dark matter particles in nature is to produce them in a laboratory. Experiments with the Large Hadron Collider (LHC) may be able to detect dark matter particles produced in collisions of the LHC proton beams. Because a dark matter particle should have negligible interactions with normal visible matter, it may be detected indirectly as (large amounts of) missing energy and momentum that escape the detectors, provided other (non-negligible) collision products are detected.{{cite journal |author1=Kane, G. |author2=Watson, S. |title=Dark Matter and LHC:. what is the Connection? |journal=Modern Physics Letters A |year=2008 |volume=23 |pages=2103–2123 |doi=10.1142/S0217732308028314 |bibcode=2008MPLA...23.2103K |arxiv=0807.2244 |issue=26|s2cid=119286980 }} Constraints on dark matter also exist from the LEP experiment using a similar principle, but probing the interaction of dark matter particles with electrons rather than quarks.{{cite journal |last1=Fox |first1=P. J. |last2=Harnik |first2=R. |last3=Kopp |first3=J. |last4=Tsai |first4=Y. |year=2011 |title=LEP Shines Light on Dark Matter |journal=Phys. Rev. D |volume=84 |issue=1 |page=014028 |arxiv=1103.0240 |bibcode=2011PhRvD..84a4028F |doi=10.1103/PhysRevD.84.014028 |s2cid=119226535}} Any discovery from collider searches must be corroborated by discoveries in the indirect or direct detection sectors to prove that the particle discovered is, in fact, dark matter.

Dark matter regularly appears as a topic in hybrid periodicals that cover both factual scientific topics and science fiction,

{{cite journal

|last=Cramer |first=John G.

|date=1 July 2003

|title=LSST – the dark matter telescope

|journal=Analog Science Fiction and Fact

|volume=123 |issue=7/8 |page=96

|issn=1059-2113 |id={{ProQuest|215342129}}

}} (Registration required)

and dark matter itself has been referred to as "the stuff of science fiction".

{{cite news

|last=Ahern |first=James

|date=16 February 2003

|title=Space travel: Outdated goal

|page=O 02

|work=The Record

|id={{ProQuest|425551312}}

}} (Registration required)

Mention of dark matter is made in works of fiction. In such cases, it is usually attributed extraordinary physical or magical properties, thus becoming inconsistent with the hypothesized properties of dark matter in physics and cosmology. For example:

• Dark matter serves as a plot device in the 1995 X-Files episode "Soft Light".

{{cite magazine

|first=Grace |last=Halden

|date=Spring 2015

|title=Incandescent: Light bulbs and conspiracies

|magazine=Dandelion: Postgraduate Arts Journal and Research Network

|volume=5 |number=2

|doi=10.16995/ddl.318 |doi-access=free

}}

• A dark-matter-inspired substance known as "Dust" features prominently in Philip Pullman's His Dark Materials trilogy.

{{cite book

|first1=Mary |last1=Gribbin

|first2=John |last2=Gribbin

|year=2007

|title=The Science of Philip Pullman's His Dark Materials

|publisher=Random House Children's Books

|isbn=978-0-375-83146-1

|pages=15–30

}}

• Beings made of dark matter are antagonists in Stephen Baxter's Xeelee Sequence.

{{cite journal

|first=Andrew |last=Fraknoi

|year=2019

|title=Science fiction for scientists

|journal=Nature Physics

|volume=12 |issue=9 |pages=819–820

|doi=10.1038/nphys3873 |s2cid=125376175

|url=https://www.nature.com/articles/nphys3873

}}

More broadly, the phrase "dark matter" is used metaphorically in fiction to evoke the unseen or invisible.

{{cite web

|first=Adam |last=Frank |author-link=Adam Frank

|date=9 February 2017

|title=Dark matter is in our DNA

|website=Nautilus Quarterly

|url=https://nautil.us/dark-matter-is-in-our-dna-2-236435/

|access-date=2022-12-11

}}

{{Gallery

|File:COSMOSDMmap2007.jpg|DM map by the Cosmic Evolution Survey (COSMOS) using the Hubble Space Telescope (2007){{cite web|title=First 3D map of the Universe's dark matter scaffolding|url=https://www.esa.int/Science_Exploration/Space_Science/First_3D_map_of_the_Universe_s_dark_matter_scaffolding|access-date=2021-11-23|website=www.esa.int|language=en}}{{cite journal|last1=Massey|first1=Richard|last2=Rhodes|first2=Jason|last3=Ellis|first3=Richard|last4=Scoville|first4=Nick|last5=Leauthaud|first5=Alexie|last6=Finoguenov|first6=Alexis|last7=Capak|first7=Peter|last8=Bacon|first8=David|last9=Aussel|first9=Hervé|last10=Kneib|first10=Jean-Paul|last11=Koekemoer|first11=Anton|date=January 2007|title=Dark matter maps reveal cosmic scaffolding|url=https://www.nature.com/articles/nature05497|journal=Nature|language=en|volume=445|issue=7125|pages=286–290|doi=10.1038/nature05497 |pmid=17206154|arxiv=astro-ph/0701594|bibcode=2007Natur.445..286M|s2cid=4429955|issn=1476-4687}}

|File:CFHTLenSDMmap2012.jpg|DM map by the CFHT Lensing Survey (CFHTLenS) using the Canada–France–Hawaii Telescope (2012){{cite web|title=News CFHT - Astronomers reach new frontiers of dark matter|url=https://www.cfht.hawaii.edu/en/news/CFHTLens/|access-date=2021-11-26|website=www.cfht.hawaii.edu}}{{cite journal|last1=Heymans|first1=Catherine|last2=Van Waerbeke|first2=Ludovic|last3=Miller|first3=Lance|last4=Erben|first4=Thomas|last5=Hildebrandt|first5=Hendrik|last6=Hoekstra|first6=Henk|last7=Kitching|first7=Thomas D.|last8=Mellier|first8=Yannick|last9=Simon|first9=Patrick|last10=Bonnett|first10=Christopher|last11=Coupon|first11=Jean|date=2012-11-21|title=CFHTLenS: the Canada–France–Hawaii Telescope Lensing Survey: CFHTLenS|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=427|issue=1|pages=146–166|arxiv=1210.0032|doi=10.1111/j.1365-2966.2012.21952.x|doi-access=free |s2cid=24731530}} (COSMOS map at the center)

|File:KiDSDMmap2015.gif|DM map by the Kilo-Degree Survey (KiDS) using the VLT Survey Telescope (2015){{cite web|title=KiDS|url=https://kids.strw.leidenuniv.nl/pr_july2015.php|access-date=2021-11-27|website=kids.strw.leidenuniv.nl}}{{cite journal|last1=Kuijken|first1=Konrad|last2=Heymans|first2=Catherine|last3=Hildebrandt|first3=Hendrik|last4=Nakajima|first4=Reiko|last5=Erben|first5=Thomas|last6=Jong|first6=Jelte T. A.|last7=Viola|first7=Massimo|last8=Choi|first8=Ami|last9=Hoekstra|first9=Henk|last10=Miller|first10=Lance|last11=van Uitert|first11=Edo|date=10 October 2015|title=Gravitational lensing analysis of the Kilo-Degree Survey|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=454|issue=4|pages=3500–3532|arxiv=1507.00738|doi=10.1093/mnras/stv2140|doi-access=free |issn=0035-8711}}

|File:HSCSDMmap2018.gif|DM map by the Hyper Suprime-Cam Survey (HSCS) using the Subaru Telescope (2018){{cite web|last=University|first=Carnegie Mellon|date=26 September 2018|title=Hyper Suprime-Cam Survey Maps Dark Matter in the Universe - News - Carnegie Mellon University|url=http://www.cmu.edu/news/stories/archives/2018/october/dark-matter-survey.html|url-status=live|website=www.cmu.edu|language=en|archive-url=https://web.archive.org/web/20200907194216/https://www.cmu.edu/news/stories/archives/2018/october/dark-matter-survey.html |archive-date=7 September 2020 }}{{cite journal|last1=Hikage|first1=Chiaki|last2=Oguri|first2=Masamune|last3=Hamana|first3=Takashi|last4=More|first4=Surhud|last5=Mandelbaum|first5=Rachel|last6=Takada|first6=Masahiro|last7=Köhlinger|first7=Fabian|last8=Miyatake|first8=Hironao|last9=Nishizawa|first9=Atsushi J|last10=Aihara|first10=Hiroaki|last11=Armstrong|first11=Robert|date=2019-04-01|title=Cosmology from cosmic shear power spectra with Subaru Hyper Suprime-Cam first-year data|url=https://academic.oup.com/pasj/article/doi/10.1093/pasj/psz010/5370019|journal=Publications of the Astronomical Society of Japan|language=en|volume=71|issue=2|page=43|arxiv=1809.09148|doi=10.1093/pasj/psz010|issn=0004-6264}}

|File:DESDMmap2021.png|DM map by the Dark Energy Survey (DES) using the Víctor M. Blanco Telescope (2021){{cite journal|last1=Jeffrey|first1=N|last2=Gatti|first2=M|last3=Chang|first3=C|last4=Whiteway|first4=L|last5=Demirbozan|first5=U|last6=Kovacs|first6=A|last7=Pollina|first7=G|last8=Bacon|first8=D|last9=Hamaus|first9=N|last10=Kacprzak|first10=T|last11=Lahav|first11=O|date=2021-06-25|title=Dark Energy Survey Year 3 results: Curved-sky weak lensing mass map reconstruction|url=https://academic.oup.com/mnras/article/505/3/4626/6287258|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=505|issue=3|pages=4626–4645|arxiv=2105.13539|doi=10.1093/mnras/stab1495|doi-access=free|issn=0035-8711}}{{cite journal|last=Castelvecchi|first=Davide|date=2021-05-28|title=The most detailed 3D map of the Universe ever made|url=http://www.nature.com/articles/d41586-021-01466-1|journal=Nature|language=en|pages=d41586–021–01466-1|doi=10.1038/d41586-021-01466-1|pmid=34050347|s2cid=235242965|issn=0028-0836}}

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{{columns-start}}

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; Related theories

• {{annotated link|Dark energy}}
• {{annotated link|Conformal gravity}}
• Density wave theory – A theory in which waves of compressed gas, which move slower than the galaxy, maintain galaxy's structure
• {{annotated link|Entropic gravity}}
• {{annotated link|Dark radiation}}
• {{annotated link|Massive gravity}}
• {{annotated link|Unparticle physics}}

; Experiments

• {{annotated link|DEAP}}, a search apparatus
• {{annotated link|LZ experiment}}, large underground dark matter detector
• {{annotated link|Dark Matter Particle Explorer|abbreviation=DAMPE}}, a space mission
• {{annotated link|General antiparticle spectrometer}}
• {{annotated link|MultiDark}}, a research program
• {{annotated link|Illustris project}}, astrophysical simulations
• {{annotated link|Future Circular Collider}}, a particle accelerator research infrastructure

{{column}}

; Dark matter candidates

• {{annotated link|Feebly Interacting Particles}}
• {{annotated link|Light dark matter}}
• {{annotated link|Mirror matter}}
• {{annotated link|Exotic matter}}
• {{annotated link|Neutralino#Relationship to dark matter|Neutralino}}
• {{annotated link|Dark galaxy}}
• {{annotated link|Scalar field dark matter}}
• {{annotated link|Self-interacting dark matter}}
• {{annotated link|Weakly interacting massive particle|abbreviation=WIMP}}
• Weakly interacting slim particle (WISP){{snd}}Low-mass counterpart to WIMP
• {{annotated link|Strongly interacting massive particle|abbreviation=SIMP}}
• {{annotated link|Chameleon particle}}

; Other

• {{annotated link|Galactic Center GeV excess}}
• Luminiferous aether – A once theorized invisible and infinite material with no interaction with physical objects, used to explain how light could travel through a vacuum (now disproven)

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{{notelist|1}}

{{reflist|25em}}

• {{cite book |last=Ferreras |first=Ignacio |title=Fundamentals of Dark Matter |year=2025 |publisher=UCL Press |isbn=9781800084704 |url=https://uclpress.co.uk/book/fundamentals-of-dark-matter/}}
• {{cite book |last1=Freeman |first1=Ken |title=In Search of Dark Matter |last2=MacNamara |first2=Geoff |date=2006 |publisher=Springer/Praxis |isbn=978-0-387-27616-8 |series=Springer-Praxis Books in Popular Astronomy |location=Berlin, Springer, Chichester}}
• {{cite book |editor-last1=Kimball |editor-first1=Derek |editor-last2=Bibber |editor-first2=Karl |title=The Search for Ultralight Bosonic Dark Matter |year=2023 |url=https://link.springer.com/book/10.1007/978-3-030-95852-7 |isbn=978-3-030-95852-7 |publisher=Springer Nature}}
• {{cite book |last=Sanders |first=Robert H. |title=The Dark Matter Problem: A historical perspective |date=2010 |publisher=Cambridge University Press |isbn=978-0-511-77357-0 |location=Cambridge, New York}}
• {{cite book |last1=Overduin |first1=James M. |title=Dark Sky, Dark Matter |last2=Wesson |first2=Paul S. |date=2003 |publisher=Institute of Physics |isbn=978-0-7503-0684-3 |series=Series in Astronomy and Astrophysics |location=Bristol}}
• {{cite book |last=Bertone |first=Gianfranco |title=Particle Dark Matter: Observations, models and searches |date=2010 |publisher=Cambridge University Press |isbn=978-0-521-76368-4 |location=Cambridge}}
• {{cite book |last=Panek |first=Richard |title=The 4 Percent Universe: Dark matter, dark energy, and the race to discover the rest of reality |date=2011 |publisher=Houghton Mifflin Harcourt |isbn=978-0-618-98244-8 |location=Boston}}
• Weiss, Rainer, (July/August 2023) "The Dark Universe Comes into Focus" Scientific American, vol. 329, no. 1, pp. 7–8.

{{Commons category|Dark matter}}

• {{cite AV media |url=https://www.ias.edu/video/the-fifth-element |title=Lecture on dark matter |author=Tremaine, Scott |publisher=IAS |medium=Video }}
• {{cite web |last1=Gray |first1=Meghan |title=Dark Matter |url=http://www.sixtysymbols.com/videos/darkmatter.htm |website=Sixty Symbols |editor-link=Brady Haran |editor=Haran, Brady |publisher=University of Nottingham |author2=Merrifield, Mike |author3=Copeland, Ed |date=2010}}

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