Dark Matter: Its NatureENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSWhat is the stuff that makes up most of the mass in theuniverse? The answer to this simple-sounding questionis far from obvious, and actually presents one of thegreatest unsolved mysteries of astrophysics, cosmology,and elementary particle physics. With conventionalastronomical methods one can only see the ‘luminousmatter’, notably in the form of stars, which reveals itspresence by the emission of light. On the other hand, onecan determine the gravitating mass of various systemssuch as spiral galaxies or galaxy clusters from theirdynamical properties, and one finds a huge discrepancyrelative to luminous matter. Assuming the usual law ofgravity, one is led to conclude that there are largeamounts of ‘dark matter’, a term first introduced by FRITZZWICKYin his seminal paper of 1933 where he studied thedynamics of GALAXY CLUSTERS.The physical properties of dark matter can be con-strained by several powerful astrophysical and cosmo-logical arguments which disfavor ‘baryonic matter’ as amain constituent. This term refers to hydrogen, helium,and the heavier elements which, besides electrons, con-sist of protons and neutrons, falling into the ‘baryon’ cat-egory of ELEMENTARY PARTICLES. But some new form ofmatter appears to hold galaxies and galaxy clusters grav-itationally together! The most popular explanation is thatof ‘particle dark matter’ which goes back to Cowsik andMcClelland who speculated in 1973 that neutrinos couldplay this role. However, while recent experiments indi-cate that neutrinos do have mass—which has been anopen question for decades—it looks impossible toattribute all of the dark matter to these weakly interact-ing particles.One is thus led to postulate hitherto undetectedelementary particles for the cosmological dark matter.On the other hand, there are already independent par-ticle-theory motivations for certain new particleswhich could well play this role. The dark-matter prob-lem thus provides one of several links between particlephysics and cosmology; it is a key ingredient of‘astroparticle physics’, or alternatively ‘PARTICLE ASTRO-PHYSICS’. The laws of the microcosm of elementary par-ticles and the macrocosm of the largest structures in theuniverse, inner space and outer space, are closely inter-twined!Perhaps the most remarkable development of the1990s is that the physics of dark matter has turned into atruly experimental science. If any of the popular specula-tions about the nature of dark matter are correct, thismysterious stuff may well turn up in one of the currentor near-future direct search experiments.In the following, the astrophysical motivation forthe reality of dark matter and the most important astro-physical constraints on its nature will be discussed. Well-motivated candidates, current search strategies and preliminary results will then be reviewed.Dynamical evidenceRotation curves of spiral galaxiesWhy are astronomers so sure that there are largeamounts of dark matter lurking everywhere in the uni-verse? The flat rotation curves of SPIRAL GALAXIES provideperhaps the most impressive evidence. These systemsconsist of a central bulge and a thin rotating disk. It isnatural to measure its orbital velocity as a function ofgalactocentric radius by virtue of the Doppler shifts ofspectral lines. Galaxy disks tend to contain neutralhydrogen which can be observed by its 21 cm line emis-sion, allowing one to measure the rotation curves tomuch larger radii than with optical tracers.The example of figure 1 illustrates the generalbehavior of the rotation curves. The orbital velocity risesfrom the center outward until it reaches a value of theorder of 100 km s–1, where it stays constant out to thelargest measured radii, a systematic trend already diag-nosed by Freeman in 1970. This behavior is entirely unex-pected because the surface luminosity of the disk falls offexponentially with radius, implying that the mass M ofluminous matter, mostly stars, is concentrated aroundthe galactic center. Thus one expects a Keplerian declineof the orbital speed, vrot= (GNM/r)1/2(Newton’s con-stant GN, radius r), in analogy to the planetary motionsin the solar system—see the dashed line in figure 1.The difference between the expected and measuredrotation curve is ascribed to the gravitational effect ofDark Matter: Its NatureCopyright © Nature Publishing Group 2002Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2002Dirac House, Temple Back, Bristol, BS21 6BE, UK1Figure 1. Rotation curve of the spiral galaxy NGC 6503 as estab-lished from radio observations of hydrogen gas in the disk (KBegeman et al Mon. Not. R. Astron. Soc. 249 439 (1991)). Thedashed curve shows the rotation curve expected from the diskmaterial alone, the chain curve from the dark-matter halo alone.Dark Matter: Its NatureENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSdark matter. A number of arguments suggest that thismaterial is not part of the galactic disk itself. First, in ourgalaxy the vertical distribution of stars together withtheir velocity dispersion reveals that there is no signifi-cant amount of disk dark matter. Second, a thin self-gravitating disk is dynamically unstable. Third, thehydrogen is vertically more extended than would beexpected if all of the gravitating matter were in the disk,especially at large radii (‘hydrogen flaring’).An overall picture of spiral galaxies emerges wherethe bulge and disk are dynamically subdominant com-ponents immersed in a huge dark-matter halo. It is notcrucial that this halo be strictly spherical; it may well besomewhat oblate or even triaxial.For the direct detection of dark matter, our ownMilky Way is the most interesting galaxy. Its rotationcurve conforms to the standard picture with an approxi-mate plateau value for the rotation velocity of 220 km s–1.The dark-matter density in the solar neighbourhoodimplied by models of the halo is 300MeVcm–3withinabout a factor of two, i.e. roughly the mass equivalent ofa hydrogen atom per 3 cm3. (Note that the atomic massunit corresponds to 931.5 MeV.)Cosmic density contributionThe contribution ρof a given matter component to theoverall density of the universe is usually expressed interms of the ‘omega parameter’ Ω=ρ/ρcrit. Here, ρcrit=3H20/(8πGN) = h21.88×10–29g cm–3is the critical densi-ty with H0the present-day cosmic