Collapsed Dark Matter Structures
  The distributions of dark matter and baryons in the Universe are known to be
very different: the dark matter resides in extended halos, while a significant
fraction of the baryons have radiated away much of their initial energy and
fallen deep into the potential wells. This difference in morphology leads to
the widely held conclusion that dark matter cannot cool and collapse on any
scale. We revisit this assumption, and show that a simple model where dark
matter is charged under a "dark electromagnetism" can allow dark matter to form
gravitationally collapsed objects with characteristic mass scales much smaller
than that of a Milky Way-type galaxy. Though the majority of the dark matter in
spiral galaxies would remain in the halo, such a model opens the possibility
that galaxies and their associated dark matter play host to a significant
number of collapsed substructures. The observational signatures of such
structures are not well explored, but potentially interesting.
