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We reconsider Sommerfeld-enhanced annihilation of dark matter (DM) intoleptons to explain PAMELA and Fermi electron and positron observations, inlight of possible new effects from substructure. There is strong tensionbetween getting a large enough lepton signal while respecting constraints onthe fluxes of associated gamma rays. We first show that these constraintsbecome significantly more stringent than in previous studies when thecontributions from background e^+ e^- are taken into account, so much so thateven cored DM density profiles are ruled out. We then show how DM annihilationswithin subhalos can get around these constraints. Specifically, if most of theobserved lepton excess comes from annihilations in a nearby (within 1 kpc)subhalo along a line of sight toward the galactic center, it is possible tomatch both the lepton and gamma ray observations. We demonstrate that this canbe achieved in a simple class of particle physics models in which the DMannihilates via a hidden leptophilic U(1) vector boson, with explicitlycomputed Sommerfeld enhancement factors. Gamma ray constraints on the main haloannihilations (and CMB constraints from the era of decoupling) require theannihilating component of the DM to be subdominant, of order 10^-2 of the totalDM density.

Overcoming Gamma Ray Constraints with Annihilating Dark Matter in Milky Way Subhalos