  We calculate the early universe evolution of perturbations in the dark matter
energy density in the context of simple dark sector models containing a GeV
scale light mediator. We consider the case that the mediator is long lived,
with lifetime up to a second, and before decaying it temporarily dominates the
energy density of the universe. We show that for primordial perturbations that
enter the horizon around this period, the interplay between linear growth
during matter domination and collisional damping can generically lead to a
sharp peak in the spectrum of dark matter density perturbation. As a result,
the population of the smallest DM halos gets enhanced. Possible implications of
this scenario are discussed.
