Thermal dark matter that couples more strongly to electrons and photons than to neutrinos will heat the electron-photon plasma relative to the neutrino background if it becomes nonrelativistic after the neutrinos decouple from the thermal background .
This results in a reduction in N _ { eff } below the standard-model value , a result strongly disfavored by current CMB observations .
Taking conservative lower bounds on N _ { eff } and on the decoupling temperature of the neutrinos , we derive a bound on the dark matter particle mass of m _ { \chi } > 3 - 9 MeV , depending on the spin and statistics of the particle .
For p -wave annihilation , our limit on the dark matter particle mass is stronger than the limit derived from distortions to the CMB fluctuation spectrum produced by annihilations near the epoch of recombination .