Cosmology’s widely accepted “concordance model” presumes that some still unknown form of dark matter overwhelms the cosmic abundance of ordinary baryonic matter by a factor of about six. Powerful evidence of dark matter has been accumulating since the 1930s from an impressive variety of observational realms: gas temperature and the random motion of galaxies in large galaxy clusters, galaxy rotation, the distribution of clusters, the abundances of the lightest elements, the anisotropy of the microwave background, and the red-shifts of distant supernovae.
In all those cases, the reasoning that leads to dark matter assumes that gravitation on galactic and larger scales is correctly described by general relativity and, in the nonrelativistic limit, by Newtonian gravity. But some cosmologists argue that imagining modifications of general relativity on large scales is no more speculative than postulating dark matter in the absence of laboratory evidence. Mordehai Milgrom’s 23-year-old phenomenological modification of Newtonian dynamics...
