  Self-interacting dark matter has been proposed as a solution to small scale
problems in cosmological structure formation, and hints of dark matter self
scattering have been observed in mergers of galaxy clusters. One of the
simplest models for self-interacting dark matter is a particle that is charged
under dark electromagnetism, a new gauge interaction analogous to the usual
electromagnetic force, but operating on the dark matter particle instead of the
visible particles. In this case, the collisional behaviour of dark matter is
primarily due to the formation of collisionless shocks, that should affect the
distribution of DM in merging galaxy clusters. We evaluate the time and length
scales of shock formation in cluster mergers, and discuss the implications for
modelling charged dark matter in cosmological simulations.
