Cosmology'swidely 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 redshifts of distant supernovae.