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Over the past few decades, an anomalous 511 keV gamma-ray line has beenobserved from the centre of the Milky Way. Dark matter (DM) in the form oflight weakly interacting massive particles (WIMPs) annihilating intoelectron-positron pairs has been one of the leading hypotheses of the observedemission. Given the small required cross section, a further coupling to lighterparticles is required to produce the correct relic density. Here, we deriveconstraints from the Planck satellite on light WIMPs that were in equilibriumwith either the neutrino or electron sector in the early universe. For theneutrino sector, we obtain a lower bound on the WIMP mass of 4 MeV for a realscalar and 10 MeV for a Dirac fermion DM particle, at 95% CL. For the electronsector, we find even stronger bounds of 7 and 11 MeV, respectively. Using theseresults, we show that, in the absence of additional ingredients such as darkradiation, the light thermally produced WIMP explanation of the 511 keV excessis strongly disfavoured by the latest cosmological data. This suggests anunknown astrophysical or more exotic DM source of the signal.

Ruling out the light WIMP explanation of the galactic 511 keV line