Dark matter might be dissipative, arising from a hidden sector with a massless dark photon. Mirror dark matter is the theoretically most constrained model of this kind, where the hidden sector Lagrangian is exactly isomorphic to that of the standard model. This kind of dark matter has rather nontrivial galaxy halo dynamics, which is very different to the collisionless cold dark matter scenario often considered in the literature. In this talk I shall discuss some aspects of this dynamics, as it is currently understood, and also discuss means of testing this type of dark matter in direct detection experiments.