  The observed matter in the universe accounts for just 5 percent of the
observed gravity. A possible explanation is that Newton's and Einstein's
theories of gravity fail where gravity is either weak or enhanced. The modified
theory of Newtonian dynamics (MOND) reproduces, without dark matter,
spiral-galaxy orbital motions and the relation between luminosity and rotation
in galaxies, although not in clusters. Recent extensions of Einstein's theory
are theoretically more complete. They inevitably include dark fields that seed
structure growth, and they may explain recent weak lensing data. However, the
presence of dark fields reduces calculability and comes at the expense of the
original MOND premise -- that the matter we see is the sole source of gravity.
Observational tests of the relic radiation, weak lensing, and the growth of
structure may distinguish modified gravity from dark matter.
