  We present a bi-metric theory of gravity containing a length scale of
galactic size. For distances less than this scale the theory satisfies the
standard tests of General Relativity. For distances greater than this scale the
theory yields an effective gravitational constant much larger than the locally
observed value of Newton's constant. The transition from one regime to the
other through the galactic scale can explain the observed rotation curves of
galaxies and hence the effects normally attributed to the presence of dark
matter. Phenomena on an extragalactic scale such as galactic clusters and the
expansion of the universe are controlled by the enhanced gravitational
coupling. This provides an explanation of the missing matter normally invoked
to account for the observed value of Hubble's constant in relation to observed
matter.
