Measurements of cosmic-ray electron and positron fluxes by PAMELA and
Fermi experiments may have indicated the existence of annihilating
dark matter with large annihilation cross section. I will discuss
various aspects of annihilating dark matter in cosmology and
astrophysics. In particular, I show that BBN and CMB give the severest
bounds on the annihilating cross section, which would kill most models
even if we consider velocity-dependent cross section, motivated by
Sommerfeld or Breit-Wigner enhancement mechanisms.