Breit-Wigner enhancement of dark matter annihilation

We point out that annihilation of dark matter in the galactic halo can be enhanced relative to that in the early Universe due to a Breit-Wigner tail, if the dark matter annihilates through a pole just below the threshold. This provides a new explanation to the boost factor which is suggested by the recent data of the PAMELA, ATIC and PPB-BETS cosmic ray experiments.