Dark Matter in Groups and Clusters of Galaxies

We compare the characteristics of stellar populations with those of dark halos. Dark matter around galaxies, and in groups, clusters and voids is discussed. Modern data suggest that the overall density of matter in the Universe is ΩM = 0.3 ± 0.1, about 80 % of this matter is nonbaryonic dark matter, and about 20 % is baryonic, mostly in the form of hot intra-cluster and intragroup gas, the rest in stellar populations of galaxies. All bright galaxies are surrounded by dark matter halos of external radii 200− 300 kpc; halos consist mostly of non-baryonic matter with some mixture of hot gas. The Universe is dominated by dark energy (cosmological constant) term. Dark matter dominates in the dynamical evolution of galaxies in groups and clusters. 1. History of the dark matter concept The story of dark matter is a classical example of a scientific revolution (Kuhn 1970, Tremaine 1987). It is impossible in this review talk to discuss all aspects of dark matter. We start with a historical introduction, followed by a comparison of ordinary stellar populations and the nature of dark matter. Thereafter we consider dark matter in galaxies, groups and clusters of galaxies, and in voids; we also discuss the mean density of matter in the Universe. First hints on the presence of a mass paradox in galaxies and clusters of galaxies came over 60 years ago. Oort (1932) noticed that there may exist a discrepancy between the dynamical estimate of the local density of matter in the Solar neighborhood in the Galaxy, and the density of luminous matter. Known stellar populations may be insufficient to explain the vertical gravitational attraction in the Galaxy which causes motions of stars perpendicular to the plane of the Galaxy. Zwicky (1933) measured radial velocities of galaxies in the Coma cluster and found that the mass of the cluster exceeds the summed mass of its galaxies more than tenfolds. These studies raise two problems, the one of the local dark matter in the disk of the Galaxy, and the global dark matter penetrating clusters of galaxies. In the 1930s astronomers were very busy to understand the evolution of stars, and dark matter problems escaped the attention of the astronomical community. The next essential step in the dark matter story was made by Kahn & Woltjer (1959). They noticed that the Andromeda galaxy and our Galaxy approach each other, whereas almost all other galaxies recede from us. The total mass of the Local Group, inferred from ascribing the approach velocity to mutual attrac-