Cosmological constraints on dark matter models with velocity-dependent   annihilation cross section
  We derive cosmological constraints on the annihilation cross section of dark matter with velocity-dependent structure, motivated by annihilating dark matter models through Sommerfeld or Breit-Wigner enhancement mechanisms. In models with annihilation cross section increasing with decreasing dark matter velocity, big-bang nucleosynthesis and cosmic microwave background give stringent constraints. 
Junji Hisano, Masahiro Kawasaki, Kazunori Kohri, Takeo Moroi, Kazunori   Nakayama, Toyokazu Sekiguchi
hep-ph astro-ph.CO