  One of the prime pieces of evidence for dark matter is the observation of
large overdense regions in the universe. Since we know from the cosmic
microwave background that the regions that contained the most baryons when the
universe was ~400,000 years old were overdense by only one part in ten
thousand, perturbations had to have grown since then by a factor greater than
$(1+z_*)\simeq 1180$ where $z_*$ is the epoch of recombination. This enhanced
growth does not happen in general relativity, so dark matter is needed in the
standard theory. We show here that enhanced growth can occur in alternatives to
general relativity, in particular in Bekenstein's relativistic version of
MOdified Newtonian Dynamics (MOND). The vector field introduced in that theory
for a completely different reason plays a key role in generating the
instability that produces large cosmic structures today.
