For the naked eye the sky seems to be dominated by stars, with an occasional nebulae here and there. At the start of this century, the large-scale distribution of matter was generally taken to mean the distribution of the stars in the Milky Way galaxy (MW hereafter). Although it was suggested that the observed nebulae could be other ``island universes'', other galaxies of stars outside the realm of the observed stars, it was not until the extensive observations done by Hubble that this new world picture was confirmed. The observations of stars seemed to reach the edge of the local stars while the observations of galaxies gave no evidence of any edge. Current observations done by, for example, the Hubble Space Telescope (HST), show that galaxies exist at redshifts roughly equivalent to distances as large as 12 million light years.
Matter in the Universe seems to be mainly clumped together in galaxies of various sizes, ranging from dwarf galaxies of about 1-3 kpc to giant galaxies of 30-50 kpc (in visual size). These galaxies are again clumped together in groups, which in turn gathers in clusters. There are also indications of clusters grouping in superclusters. Clustering of galaxies into groups is typical for the distribution of galaxies in the Universe. Studies show that less than 10-20 per cent of the galaxies do not belong to any group, often referred to as ``field'' galaxies.
Groups of galaxies may consist of about 5-100 galaxies, with sizes ranging from a just below a hundred kiloparsec to 1 or 2 megaparsec, while clusters consists of more than 100 galaxies, with typical size of about a few megaparsec. Just like galaxies, one may approximate clusters and groups as gravitationally bound systems of effectively point particles, which, as we shall see in Section 3.1, is one of the assumptions that the AVP is based on. Clusters, on the other hand, are also known to contain a matter element that is not being taken account for in the AVP method; namely, a very hot intracluster gas with a mass about equal to the mass in galaxies. [More on the distribution of matter in]Pad
The distribution of matter on scales ranging from atomic level to superclusters seems to be very irregular. This may appear to be in conflict with Einstein's cosmological principle, which states that the universe is homogeneous and isotropic on the large-scale average. But, on large enough scales the principle seems to be valid: observations on scales larger than 100 Mpc seem to indicate a reasonably uniform distribution. Such observations, for example done by Hubble, and Einstein's credibility has almost resulted in the cosmological principle becoming a physical principle.
An assumption used above is that the mass in the Universe is traced by light--is this so? A number of observations not directly based on the luminous matter; i.e. dynamics of galaxies in clusters, rotational curve of disc galaxies, and gravitational lenses gives a negative answer to this question. The density parameter due to luminous matter in the universe is [page 29]Pad, while studies of the dynamics of galaxies in clusters indicates a value of about 10 per cent of the critical Einstein-de Sitter value ( ). This means that a factor of ten of the mass is not directly observable, often referred to as dark matter. Some observations and theories even suggests that yet another factor of ten of the mass in the Universe might be dark, located on a scale larger than the systems of galaxies, bringing the total mass density to . There is a lot of controversy on the latter. Some theories even imply that this large-scale dark matter component is not matter per se, but is rather being caused by something acting like a cosmological constant. Such a term would satisfy the condition from inflation theory that space curvature is negligible small, and would have practically no effect on the dynamics of groups and clusters of galaxies.
The question of what constitutes the dark matter is presently one of the most important one in cosmology. One usually distinguish between baryonic and non-baryonic dark matter: faint stars, jupiters, black holes, and inter-galactic gas are examples of the former; while weakly-interacting massive particles (WIMPs) and topological defects of gauge fields arising from quantum symmetries are examples of the latter. Non-baryonic matter can be broadly be grouped into two kinds; ``hot'' and ``cold''. These adjectives indicate how fast a dark matter particle was moving when it decoupled from the baryonic matter (relativistic or not). Massive neutrinos are an example of hot dark matter (HDM) particles, and axions and primordial black holes are examples of cold dark matter (CDM). One sometimes also talk about a mixture of HDM and CDM: mixed dark matter (MDM).
The dark mass in disc galaxies is assumed to be in spheroidal halos, extending to much larger radii than the optical disc. A number of observations have confirmed the existence of massive dark matter halos around all types of galaxies. Candidates for the dark halos are some compact baryonic objects, usually called ``Massive Astrophysical Compact Halo Objects'' (MACHO), and/or WIMPs. The halos can have played a crucial role in the evolution of galaxies, especially if their size was so large that they frequently overlapped when galaxies were in close passage of each other.