  We consider a dark sector consisting of dark matter that is a Dirac fermion
and a scalar mediator. This model has been extensively studied in the past. If
the scalar couples to the dark matter in a parity conserving manner then dark
matter annihilation to two mediators is dominated by the P-wave channel and
hence is suppressed at very low momentum. The indirect detection constraint
from the anisotropy of the Cosmic Microwave Background is usually thought to be
absent in the model because of this suppression. In this letter we show that
dark matter annihilation to bound states occurs through the S-wave and hence
there is a constraint on the parameter space of the model from the Cosmic
Microwave Background.
