Figure 1

Figure 1: Summary of the empirical roots of the missing mass problem (below line) and the generic possibilities for its solution (above line). Illustrated lines of evidence include the approximate flatness of the rotation curves of spiral galaxies, gravitational lensing in a cluster of galaxies, and the growth of large-scale structure from an initially very-nearly–homogeneous early Universe. Other historically-important lines of evidence include the Oort discrepancy, the need to stabilize galactic disks, motions of galaxies within clusters of galaxies and the hydrodynamics of hot, X-ray emitting gas therein, and the apparent excess of gravitating mass density over the mass density of baryons permitted by Big-Bang nucleosynthesis. From these many distinct problems grow several possible solutions. Generically, the observed discrepancies either imply the existence of dark matter, or the necessity to modify dynamical laws. Dark matter could, in principle, be any combination of non-luminous baryons and/or some non-baryonic form of mass-like neutrinos (hot dark matter) or some new particle, whose mass makes it dynamically cold or perhaps warm. Alternatively, the observed discrepancies might point to the need to modify the equation of gravity that is employed to infer the existence of dark matter, or perhaps some other fundamental dynamical assumption like the equivalence of inertial mass and gravitational charge. Many specific ideas of each of these types have been considered over the years. Note that none of these ideas are mutually exclusive, and that some form or the other of dark matter could happily cohabit with a modification of the gravitational law, or could even be itself the cause of an effective modification of the gravitational law. Question marks on some tree branches represent the fruit of ideas yet to be had. Perhaps these might also address the dark energy problem, with the most satisfactory result being a theory that would simultaneously explain the acceleration scale in the dark matter problem as well as the accelerating expansion of the Universe, and explain the coincidence of scales between these two problems, a coincidence exhibited in Section 4.1.