Precision measurements of the positron flux in cosmic ray have revealed an unexplained bump in the spectrum around E \simeq 300 \mathrm { GeV } , not clearly attributable to known astrophysical processes .
We propose annihilation of dark matter of mass m _ { \chi } = 780 \mathrm { GeV } with a late-time cross section \sigma v = 4.63 \times 10 ^ { -24 } \mathrm { cm ^ { 3 } s ^ { -1 } } as a possible source .
The nonmonotonic dependence of the annihilation rate on dark matter velocity , owing to a selective p -wave Sommerfeld enhancement , allows such a large signal from the Milky Way without violating corresponding constraints from CMB and dwarf galaxy observations .
We briefly explore other signatures of this scenario , and outline avenues to test it in future experiments .