2.1 Probes of gravitational wells

The existence of “dark matter” is inferred from astrophysical observations that probe gravitational potentials. The mass content required to provide the derived gravitational potential is then compared with the visible mass content. Several types of observation allow this to be done and in most cases the mismatch between the required mass and the observed mass is extreme. The following list summarises some of the evidence that has been accumulated:

The next four items are not really at the same level of “simple” observational evidence as those above, as they require reliance on a more convoluted path to determine masses involved. However, the first three of these have received a great deal of effort and are now heavily used as a combined strong argument in favour of the existence of “dark matter”, and indeed have resulted in a consensus view of “standard cosmology” over the past few years.

With such a large volume of evidence there can be no doubt that there is a real mystery to be unravelled here. Ideally, it would be satisfying if there were a single simple solution that explained all the above. This has proven elusive so far, but recently there has been some convergence on models that address the larger scale issues to do with the Universe as a whole, and this is discussed in the next subsection. The main aim is to establish a consensus opinion on the dark matter fraction, and more specifically the cold dark matter fraction, as this motivates most of the experimental searches for dark matter. In doing this we will see that a strong argument for a standard cosmology, with cold dark matter as one of its components, is beginning to become established. However, some issues clearly hint at aspects of the cosmology that have yet to be properly resolved, and some of these do have potentially serious implications for the cold dark matter component. These will be discussed in Section 2.3.

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