Dark matter annihilation rate with nonstandard thermal history

We consider the cosmology of a model in which the dark matter is part of a nonminimal hidden sector which is not in thermal equilibrium with the standard model particles at the time of dark matter decoupling. We show that the annihilation rate of dark matter can be either lower or higher than in standard thermal decoupling models, but no higher than a factor of about 5, unless there is a Sommerfeld enhancement. We use as a minimal example a simple model in which dark matter couples to the standard model through a light scalar intermediary that is itself unstable. This model has several notable features, and allows a natural explanation for a surplus of positrons and electrons, as has been suggested by data from PAMELA, ATIC and FERMI.