  Based on observational constraints on large scale structure and halo
structure, dark matter is generally taken to be cold and essentially
collisionless. On the other hand, given the large number of particles and
forces in the visible world, a more complex dark sector could be a reasonable
or even likely possibility. This hypothesis leads to testable consequences,
perhaps portending the discovery of a rich hidden world neighboring our own. We
consider a scenario that readily satisfies current bounds that we call
Partially Interacting Dark Matter (PIDM). This scenario contains
self-interacting dark matter, but it is not the dominant component. Even if
PIDM contains only a fraction of the net dark matter density, comparable to the
baryonic fraction, the subdominant component's interactions can lead to
interesting and potentially observable consequences. Our primary focus will be
the special case of Double-Disk Dark Matter (DDDM), in which self-interactions
allow the dark matter to lose enough energy to lead to dynamics similar to
those in the baryonic sector. We explore a simple model in which DDDM can cool
efficiently and form a disk within galaxies, and we evaluate some of the
possible observational signatures. The most prominent signal of such a scenario
could be an enhanced indirect detection signature with a distinctive spatial
distribution. Even though subdominant, the enhanced density at the center of
the galaxy and possibly throughout the plane of the galaxy can lead to large
boost factors, and could even explain a signature as large as the 130 GeV Fermi
line. Such scenarios also predict additional dark radiation degrees of freedom
that could soon be detectable and would influence the interpretation of future
data, such as that from Planck and from the Gaia satellite. We consider this to
be the first step toward exploring a rich array of new possibilities for dark
matter dynamics.
