γ-Rays from light dark matter

The idea of light dark matter provides a plausible explanation for the observed 511 keV line emission from the galactic bulge. But the processes involved also emit the continuous spectrum of gamma-rays. We present a detailed calculation of continuous spectrum for the decay model of light dark matter. Confrontation with the observed spectrum is challenging. We get the conservative mass bound m(LDM) less than or similar to 20 MeV for the decay model, and find that better understanding is needed for the diffuse gamma-rays from the galactic bulge in the energy range 20 keV - 20 MeV.