  We consider a class of models in which thermal dark matter is lighter than an
MeV. If dark matter thermalizes with the Standard Model below the temperature
of neutrino-photon decoupling, equilibration and freeze-out cools and heats the
Standard Model bath comparably, alleviating constraints from measurements of
the effective number of neutrino species. We demonstrate this mechanism in a
model consisting of fermionic dark matter coupled to a light scalar mediator.
Thermal dark matter can be as light as a few keV, while remaining compatible
with existing cosmological and astrophysical observations. This framework
motivates new experiments in the direct search for sub-MeV thermal dark matter
and light force carriers.
