  In models of dark matter (DM) with Sommerfeld-enhanced annihilation, where
the annihilation rate scales as the inverse velocity, N-body simulations of DM
structure formation suggest that the local annihilation signal may be dominated
by small, dense, cold subhalos. This contrasts with the usual assumption of a
signal originating from the smooth DM halo, with much higher velocity
dispersion. Accounting for local substructure modifies the parameter space for
which Sommerfeld-enhanced annihilating DM can explain the PAMELA and Fermi
excesses. Limits from the inner galaxy and the cosmic microwave background are
weakened, without introducing new tension with substructure-dependent limits,
such as from dwarf galaxies or isotropic gamma-ray studies. With substructure,
previously excluded parameter regions with mediators of mass ~1-200 MeV are now
easily allowed. For O(MeV) mediators, subhalos in a specific range of host halo
masses may be evaporated, further suppressing diffuse signals without affecting
substructure in the Milky Way.
