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{{Short description|Open problem concerning the unknown physical nature of dark matter}}
{{Quantum book backlink|Advanced and frontier topics}}

The '''quantum dark matter problem''' is the open problem of identifying the physical nature of [[Physics:Quantum Standard Model#Limitations|dark matter]] and explaining how it fits into quantum physics, particle physics, and cosmology. Dark matter is inferred from gravitational effects that cannot be explained by visible matter alone, including galaxy rotation curves, gravitational lensing, galaxy-cluster dynamics, the cosmic microwave background, and the formation of large-scale cosmic structure.

In the standard cosmological model, ordinary matter accounts for only a small fraction of the total mass-energy content of the universe, while dark matter forms most of the matter component. The Planck mission results support a universe made of approximately ordinary matter, dark matter, and dark energy, with dark matter making up about five-sixths of all matter.<ref name="NASAPlanck">{{cite web |url=http://www.nasa.gov/mission_pages/planck/news/planck20130321.html |title=Planck Mission Brings Universe into Sharp Focus |website=NASA Mission Pages |date=21 March 2013 |access-date=1 May 2016 |archive-date=12 November 2020 |archive-url=https://web.archive.org/web/20201112001039/http://www.nasa.gov/mission_pages/planck/news/planck20130321.html }}</ref><ref name="Planck2015">{{cite journal |last1=Ade |first1=P. A. R. |display-authors=etal |title=Planck 2015 results. XIII. Cosmological parameters |journal=Astronomy & Astrophysics |year=2016 |volume=594 |issue=13 |page=A13 |doi=10.1051/0004-6361/201525830 |bibcode=2016A&A...594A..13P |arxiv=1502.01589 |s2cid=119262962 }}</ref>

The problem is quantum-relevant because most leading dark-matter candidates are hypothetical particles or quantum fields beyond the Standard Model. Examples include weakly interacting massive particles, axions, sterile neutrinos, ultralight bosons, and hidden-sector particles. Another possibility is that dark matter is made of primordial black holes, which would connect the problem to early-universe quantum fluctuations and gravitational collapse.<ref name="BertoneHooperSilk">{{cite journal |last1=Bertone |first1=Gianfranco |last2=Hooper |first2=Dan |last3=Silk |first3=Joseph |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 }}</ref><ref name="Carr24">{{cite journal |last1=Carr |first1=B. J. |last2=Clesse |first2=S. |last3=García-Bellido |first3=J. |last4=Hawkins |first4=M. R. S. |last5=Kühnel |first5=F. |title=Observational evidence for primordial black holes: A positivist perspective |journal=Physics Reports |date=26 February 2024 |volume=1054 |pages=1–68 |doi=10.1016/j.physrep.2023.11.005 |url=https://www.sciencedirect.com/science/article/pii/S0370157323003976 |issn=0370-1573 |arxiv=2306.03903 |bibcode=2024PhR..1054....1C }}</ref>

<div style="float:right; border:1px solid #e0d890; background:#fff8cc; padding:6px; margin:0 0 1em 1em; width:360px;">
[[File:Quantum_dark_matter_problem_yellow.png|340px]]
<div style="font-size:90%;">The dark matter problem asks what invisible quantum matter or compact objects produce the gravitational effects seen in galaxies and the cosmic web.</div>
</div>

== Basic idea ==

Dark matter is called "dark" because it does not emit, absorb, or reflect enough electromagnetic radiation to be observed directly with light. It is called "matter" because it behaves gravitationally like a matter component: its density dilutes as the universe expands, unlike radiation or dark energy.

The central question is not whether there is missing gravitational mass. Many independent observations indicate that there is. The harder question is what the missing component is made of, how it was produced in the early universe, and why it has not yet been detected directly in laboratory experiments.

In most modern models, dark matter is non-baryonic. This means it is not mainly made of ordinary protons, neutrons, atoms, planets, faint stars, or gas clouds. Big Bang nucleosynthesis, cosmic microwave background data, and microlensing searches strongly constrain how much ordinary baryonic matter can be hidden in compact objects.<ref name="Copi1995">{{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 }}</ref><ref name="Tisserand2007">{{cite journal |last1=Tisserand |first1=P. |display-authors=etal |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 }}</ref>

== Observational evidence ==

The evidence for dark matter is not based on one observation. It comes from several independent gravitational phenomena.

=== Galaxy rotation curves ===

Spiral galaxies rotate differently from what would be expected if their visible stars and gas contained nearly all their mass. If most of the mass were concentrated in the luminous central region, orbital velocities should decrease with distance from the galactic center. Instead, many galaxy rotation curves remain approximately flat at large radii.

This behavior suggests that galaxies are embedded in extended halos of unseen matter. Vera Rubin, Kent Ford, and other observers helped establish galaxy rotation curves as one of the major lines of evidence for dark matter.<ref name="Rubin1970">{{cite journal |last1=Rubin |first1=V. C. |last2=Ford |first2=W. K. Jr. |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 }}</ref><ref name="Roberts1975">{{cite journal |last1=Roberts |first1=Morton S. |last2=Whitehurst |first2=Robert N. |date=October 1975 |title=The rotation curve and geometry of M31 at large galactocentric distances |journal=The Astrophysical Journal |volume=201 |pages=327–346 |bibcode=1975ApJ...201..327R |doi=10.1086/153889 }}</ref><ref name="Salucci2019">{{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 |article-number=2 |doi=10.1007/s00159-018-0113-1 |arxiv=1811.08843 |bibcode=2019A&ARv..27....2S }}</ref>

=== Galaxy clusters ===

Galaxy clusters provide another major line of evidence. Their masses can be estimated from galaxy velocities, hot X-ray gas, and gravitational lensing. These methods generally indicate more mass than is visible in stars and gas.<ref name="AllenClusters">{{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 }}</ref>

The Bullet Cluster is a particularly important example. It is a collision of two galaxy clusters in which the hot gas, seen in X-rays, is separated from the main gravitational mass inferred from lensing. This separation is difficult to explain using ordinary visible matter alone and is often cited as direct empirical evidence for dark matter.<ref name="Clowe2006">{{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 }}</ref>

=== Gravitational lensing ===

Gravitational lensing measures mass by the bending of light. It does not require the mass to shine. Maps of weak and strong lensing around galaxies and clusters often reveal mass distributions that exceed or differ from the distribution of visible matter.

Lensing is especially important because it measures gravity directly. It can therefore test whether the missing mass follows visible matter or forms a separate dark component.<ref name="Refregier2003">{{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 }}</ref>

=== Cosmic microwave background ===

The cosmic microwave background contains small temperature anisotropies that encode the composition of the early universe. Ordinary matter and dark matter affect the acoustic peaks in different ways because ordinary matter interacts with radiation, while dark matter mainly contributes gravitationally.

The observed pattern of CMB anisotropies is well fitted by the Lambda-CDM model, which includes cold dark matter.<ref name="Hinshaw2009">{{cite journal |last1=Hinshaw |first1=G. |display-authors=etal |year=2009 |title=Five-year Wilkinson microwave anisotropy probe 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 }}</ref><ref name="Planck2015" />

=== Structure formation ===

Dark matter helps explain how galaxies and clusters formed. Ordinary matter in the early universe was coupled to radiation and could not collapse efficiently before recombination. Dark matter, which did not interact electromagnetically, could begin forming gravitational potential wells earlier.

These potential wells helped ordinary matter fall in later, allowing large-scale structure to form within the age of the universe. This is one reason cold dark matter is a central part of the standard cosmological model.

== Candidate particles ==

The identity of dark matter is unknown. If dark matter is made of particles, those particles must be stable or extremely long-lived, mostly invisible to electromagnetic radiation, and abundant enough to explain cosmological observations.

=== Weakly interacting massive particles ===

Weakly interacting massive particles, or WIMPs, were long among the most studied candidates. A WIMP is usually understood as a heavy particle that interacts through gravity and possibly through weak-scale interactions. Many WIMP models produce the right relic abundance through thermal freeze-out in the early universe.<ref name="Jungman1996">{{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 }}</ref>

Direct detection experiments have searched for nuclear recoils produced by WIMP scattering. Experiments such as LUX, XENON, PandaX, and LZ have placed increasingly strong limits but have not produced a confirmed WIMP detection.<ref name="Akerib2017">{{cite journal |last=Akerib |first=D. S. |display-authors=etal |title=Results from a Search for Dark Matter in the Complete LUX Exposure |journal=Physical Review Letters |volume=118 |issue=2 |article-number=021303 |date=2017 |doi=10.1103/PhysRevLett.118.021303 |pmid=28128598 |arxiv=1608.07648 |bibcode=2017PhRvL.118b1303A }}</ref><ref name="Aprile2018">{{cite journal |last=Aprile |first=E. |display-authors=etal |collaboration=XENON Collaboration |title=Dark Matter Search Results from a One Ton-Year Exposure of XENON1T |journal=Physical Review Letters |volume=121 |issue=11 |article-number=111302 |date=2018 |doi=10.1103/PhysRevLett.121.111302 |pmid=30265108 |arxiv=1805.12562 |bibcode=2018PhRvL.121k1302A }}</ref>

=== Axions ===

Axions are light hypothetical particles originally proposed to solve the strong CP problem in quantum chromodynamics. They are also natural dark-matter candidates because coherent oscillations of the axion field can behave like cold dark matter.<ref name="Peccei2008">{{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 }}</ref><ref name="PreskillAxion">{{cite journal |last1=Preskill |first1=J. |last2=Wise |first2=M. |last3=Wilczek |first3=F. |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 }}</ref><ref name="AbbottSikivie">{{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 }}</ref>

Axion searches often use strong magnetic fields to convert axions into photons. The Axion Dark Matter Experiment has reached sensitivity to important axion-model parameter ranges.<ref name="Braine2020">{{cite journal |last=Braine |first=T. |display-authors=etal |collaboration=ADMX Collaboration |title=Extended Search for the Invisible Axion with the Axion Dark Matter Experiment |journal=Physical Review Letters |volume=124 |issue=10 |article-number=101303 |date=2020 |doi=10.1103/PhysRevLett.124.101303 |pmid=32216421 |arxiv=1910.08638 |bibcode=2020PhRvL.124j1303B }}</ref>

=== Sterile neutrinos and light dark matter ===

Sterile neutrinos are hypothetical neutral fermions that would not interact through the ordinary weak interaction. Depending on their mass and production mechanism, they could behave as warm dark matter.

Other possibilities include light dark-sector particles, fuzzy dark matter, dark photons, and self-interacting dark matter. These models are motivated partly by the lack of confirmed WIMP detections and partly by small-scale structure questions in galaxies.<ref name="Bergstrom2000">{{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 }}</ref>

== Primordial black holes ==

Primordial black holes are hypothetical black holes formed in the early universe rather than from stellar collapse. They could have formed from unusually dense regions produced by early-universe fluctuations. Because they would have formed before ordinary stars, they are not ordinary baryonic objects in the usual stellar sense and can be considered dark-matter candidates.<ref name="Carr24" /><ref name="Bird2023">{{cite journal |last1=Bird |first1=Simeon |last2=Albert |first2=Andrea |last3=Dawson |first3=Will |last4=Ali-Haïmoud |first4=Yacine |last5=Coogan |first5=Adam |display-authors=5 |title=Primordial black hole dark matter |journal=Physics of the Dark Universe |date=1 August 2023 |volume=41 |article-number=101231 |doi=10.1016/j.dark.2023.101231 |arxiv=2203.08967 |bibcode=2023PDU....4101231B |s2cid=247518939 }}</ref>

Primordial black holes are constrained by microlensing, cosmic microwave background data, gravitational waves, wide binaries, and other observations. Some mass windows are strongly constrained, but the possibility that primordial black holes contribute to some or all dark matter remains actively studied.<ref name="Carr2022">{{cite journal |last1=Carr |first1=Bernard |last2=Kühnel |first2=Florian |title=Primordial black holes as dark matter candidates |journal=SciPost Physics Lecture Notes |date=2 May 2022 |article-number=48 |doi=10.21468/SciPostPhysLectNotes.48 |s2cid=238407875 |url=https://scipost.org/SciPostPhysLectNotes.48/pdf |doi-access=free |arxiv=2110.02821 }}</ref><ref name="Green2021">{{cite journal |last1=Green |first1=Anne M. |last2=Kavanagh |first2=Bradley J. |title=Primordial Black Holes as a Dark Matter Candidate |journal=Journal of Physics G: Nuclear and Particle Physics |volume=48 |issue=4 |page=043001 |date=2021 |doi=10.1088/1361-6471/abc534 |arxiv=2007.10722 |bibcode=2021JPhG...48d3001G }}</ref>

== Detection strategies ==

Dark matter searches use several complementary methods.

'''Direct detection''' experiments look for rare collisions between dark matter particles and atomic nuclei or electrons in a detector. These experiments are usually placed deep underground to reduce cosmic-ray backgrounds.

'''Indirect detection''' searches look for products of dark matter annihilation or decay, such as gamma rays, positrons, antiprotons, or neutrinos. The Galactic Center, dwarf galaxies, and galaxy clusters are common targets.<ref name="BertoneMerritt">{{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 }}</ref>

'''Collider searches''' attempt to produce dark matter particles in high-energy collisions. At the Large Hadron Collider, dark matter would usually appear indirectly as missing energy and momentum, often accompanied by visible particles or jets.<ref name="KaneWatson">{{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 }}</ref>

'''Astrophysical probes''' use stars, black holes, gravitational waves, and lensing maps to constrain dark matter. For example, ultralight bosons may be constrained by black-hole superradiance, while axions may be constrained by stellar cooling.<ref name="Raffelt1996">{{cite book |last1=Raffelt |first1=Georg G. |title=Stars as laboratories for fundamental physics: the astrophysics of neutrinos, axions, and other weakly interacting particles |date=1996 |publisher=University of Chicago Press |location=Chicago |isbn=978-0-226-70272-8 |url=https://wwwth.mpp.mpg.de/members/raffelt/mypapers/Stars.pdf }}</ref><ref name="Cardoso2018">{{cite journal |last1=Cardoso |first1=Vitor |last2=Dias |first2=Óscar J. C. |last3=Hartnett |first3=Gavin S. |last4=Middleton |first4=Matthew |last5=Pani |first5=Paolo |last6=Santos |first6=Nuno M. |title=Constraining the mass of dark photons and axion-like particles through black-hole superradiance |journal=Journal of Cosmology and Astroparticle Physics |volume=2018 |issue=3 |page=043 |year=2018 |arxiv=1801.01420 |doi=10.1088/1475-7516/2018/03/043 |bibcode=2018JCAP...03..043C }}</ref>

== Modified gravity ==

A minority view is that the observations attributed to dark matter may instead indicate that gravity itself must be modified on galactic or cosmological scales. Modified Newtonian dynamics, relativistic MOND-like theories, entropic gravity, and other approaches attempt to explain some dark-matter phenomena without new particles.

Modified gravity models can reproduce some observations, especially aspects of galaxy rotation curves. However, it is difficult for such models to explain all the evidence at once, including the cosmic microwave background, gravitational lensing, galaxy clusters, and large-scale structure.<ref name="HossenfelderMcGaugh">{{cite journal |url=https://www.scientificamerican.com/article/is-dark-matter-real/ |title=Is dark matter real? |date=August 2018 |last1=Hossenfelder |first1=Sabine |last2=McGaugh |first2=Stacy S. |journal=Scientific American |volume=319 |issue=2 |pages=36–43 |doi=10.1038/scientificamerican0818-36 |pmid=30020902 |bibcode=2018SciAm.319b..36H |s2cid=51697421 |url-access=subscription }}</ref><ref name="CarrollTrialogue">{{cite web |author=Carroll |first=Sean |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}}</ref>

== Why it remains unsolved ==

The dark matter problem remains unsolved because the gravitational evidence is strong, but the underlying substance has not been directly identified. The most conservative explanation is some form of non-baryonic matter, probably involving particles or fields beyond the Standard Model. Yet decades of experimental searches have not produced a confirmed detection.

The problem therefore sits at the boundary between cosmology, quantum field theory, particle physics, astrophysics, and gravity. A solution would reveal either a new particle or field, a new compact-object population, a modification of gravity, or some combination of these.

== Status ==

Dark matter remains one of the central open problems in modern physics. The Lambda-CDM model successfully describes a wide range of cosmological observations, but the microscopic identity of dark matter is still unknown.

The main current possibilities include WIMPs, axions, sterile neutrinos, ultralight bosons, hidden-sector particles, self-interacting dark matter, and primordial black holes. No candidate has yet been confirmed.

== See also ==

{{#invoke:PhysicsQC|tocHeadingAndList|Physics:Quantum basics/See also}}

=References=
{{reflist|3}}

== Further reading ==

* {{cite book |last=Ferreras |first=Ignacio |title=Fundamentals of Dark Matter |year=2025 |publisher=UCL Press |isbn=978-1-80008-470-4 |url=https://uclpress.co.uk/book/fundamentals-of-dark-matter/}}
* {{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 |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 |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 |doi=10.1007/978-3-030-95852-7 |bibcode=2023subd.book.....K }}
* {{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 |article-number=045002 |doi=10.1103/RevModPhys.90.045002 |arxiv=1605.04909 |bibcode=2018RvMP...90d5002B |s2cid=18596513 }}

{{Author|Harold Foppele}}

{{Sourceattribution|Dark matter|1}}

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