  Spergel and Steinhardt have recently proposed the concept of dark matter with
strong self-interactions as a means to address numerous discrepancies between
observations of dark matter halos on subgalactic scales and the predictions of
the standard collisionless dark matter picture. We review the motivations for
this scenario and discuss some recent, successful numerical tests. We also
discuss the possibility that the dark matter interacts strongly with ordinary
baryonic matter, as well as with itself. We present a new analysis of the
experimental constraints and re-evaluate the allowed range of cross-section and
mass.
