A Fourth Exception in the Calculation of Relic Abundances
  We propose that the dark matter abundance is set by the decoupling of
inelastic scattering instead of annihilations. This coscattering mechanism is
generically realized if dark matter scatters against states of comparable mass
from the thermal bath. Coscattering points to dark matter that is exponentially
lighter than the weak scale and has a suppressed annihilation rate, avoiding
stringent constraints from indirect detection. Dark matter upscatters into
states whose late decays can lead to observable distortions to the blackbody
spectrum of the cosmic microwave background.
