ABSTRACT
There is now a plethora of evidence for dark matter in the Universe. The galaxies and galaxy clusters, the bullet cluster, the largescale structure of the Universe, the Lyman alpha forest in intergalactic medium, and more importantly the Planck and WMAP results of cosmic microwave background radiation anisotropy studies, all uphold the presence of dark matter in the Universe. Although the mass-toluminosity ratio (M/L) measurements of galaxy clusters gave an idea of the amount of dark matter present in that cluster, it is the results of probing the anisotropies in the expected isotropy of cosmic microwave background (CMB) radiation that gave an estimate of dark matter content in the whole Universe. In fact, the Wilkinson Microwave Anisotropy Probe or WMAP [42], a satellite-borne experiment, and more recent data from another satellite-borne experiment, namely Planck [43] for probing CMB anisotropies, suggest that the Universe is maintaining its critical density ρc (the density required to keep it spatially flat) such that the total density is expressed as
Ω = ρρc = 1 , (8.1)
out of which the dark matter content is ΩDM ≃ 0.268 or 26.8% in comparison to the total matter content of ∼ 31.7%. In Fig. 8.1, the energy budget of the Universe is shown. The Planck and WMAP results also reveal that most of the dark matter must be non-baryonic in nature. In addition to that, the dark matter particle should be stable and electrical and color neutral. If the dark matter candidate would have electromagnetically and strongly interacted with normal matter, it would have formed isotopes of estimated abundance (n/nH) <∼ 10−10, which contradicts the present upper limit of hydrogen isotopes. Thus the dark matter candidates (or at least a large part of the possible candidates) are likely to be weakly interacting.
