Ruling out the light weakly interacting massive particle explanation of the Galactic 511 keV line

Over the past few decades, an anomalous 511 keV gamma-ray line has been observed from the center of the Milky Way. Dark matter (DM) in the form of light (≲10  MeV) weakly interacting massive particles (WIMPs) annihilating into electron-positron pairs has been one of the leading hypotheses of the observed emission. Given the small required cross section, ⟨σv⟩∼10⁻³⁰  cm³ s⁻¹, a further coupling to lighter particles is required to produce the correct relic density. Here, we derive constraints from the Planck satellite on light WIMPs that were in equilibrium with either the neutrino or electron sector in the early universe. For the neutrino sector, we obtain a lower bound on the WIMP mass of 4 MeV for a real scalar and 10 MeV for a Dirac fermion DM particle, at 95% C.L. For the electron sector, we find even stronger bounds of 7 and 11 MeV, respectively. Using these results, we show that, in the absence of additional ingredients such as dark radiation, the light thermally produced WIMP explanation of the 511 keV excess is strongly disfavored by the latest cosmological data. This suggests an unknown astrophysical or more exotic DM source of the signal.