Self-interacting hidden sector dark matter, small scale galaxy structure anomalies, and a dark force.

The short distance behavior of dark matter (DM) at galaxy scales exhibits several features not explained by the typical CDM with velocity-independent cross-section. We discuss a particle physics model with a hidden sector interacting feebly with the visible sector where a dark fermion self-interacts via a dark force with a light dark photon as the mediator. We study coupled Boltzmann equations involving two temperatures, one for each sector. We fit the velocity-dependent DM cross-section to the data from scales of dwarf galaxies to clusters consistent with relic density constraint. The modified gravity dynamics, which is an alternative to dark matter, as an explanation of anomalies is noted.