  Recent observations from PAMELA, FERMI, and ATIC point to a new source of
high energy cosmic rays. If these signals are due to annihilating dark matter,
then the annihilation cross section in the present day must be substantially
larger than that necessary for thermal freeze-out in the early universe. A new
force, mediated by a particle of mass O(100 MeV), leading to a velocity
dependent annihilation cross section - a `Sommerfeld enhancement' - has been
proposed as a possible explanation. We point out that such models necessarily
increase the dark matter (DM) self-scattering cross section, and use
observational bounds on the amount of DM-DM scattering allowed in various
astrophysical systems to place constraints on the mass and couplings of the
light mediator.
