In this Letter we study pair annihilation processes of dark matter (DM) in the Universe, in the case that the DM is an electroweak gauge nonsinglet. In the current Universe, in which the DM is highly nonrelativistic, the nonperturbative effect may enhance the DM annihilation cross sections, especially for that to two photons, by several orders of magnitude. We also discuss sensitivities in future searches for anomalous $\gamma$ rays from the galactic center, which originate from DM annihilation.

• Received 25 July 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.031303