The unambiguous detection of Galactic dark matter annihilation would unravel one of the most outstanding puzzles in particle physics and cosmology. Recent observations have motivated models in which the annihilation rate is boosted by the Sommerfeld effect, a non-perturbative enhancement arising from a long range attractive force. Here we apply the Sommerfeld correction to Via Lactea II, a high resolution N-body simulation of a Milky-Way-size galaxy, to investigate the phase-space structure of the Galactic halo. We show that the annihilation luminosity from kinematically cold substructure can be enhanced by orders of magnitude relative to previous calculations, leading to the prediction of γ-ray fluxes from up to hundreds of dark clumps that should be detectable by the Fermi satellite. In the standard cold dark matter (CDM) paradigm of structure formation, a weakly interacting massive particle (WIMP) of mass mχ ∼ 100 GeV-10 TeV ceases to annihilate when the universe cools to a temperature of Tf ∼ mχ/20, about one nano-second after the Big Bang. A thermally-averaged cross-section at freeze-out of 〈σv〉0 ≈ 3 × 10 −26 cm 3 s −1 results in a reli
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