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The origin of both the diffuse high-latitude MeV gamma-ray emission and the511 keV line flux from the Galactic bulge are uncertain. Previous studies haveinvoked dark matter physics to independently explain these observations, thoughas yet none has been able to explain both of these emissions within thewell-motivated framework of Weakly-Interacting Massive Particles (WIMPs). Herewe use an unstable WIMP dark matter model to show that it is in fact possibleto simultaneously reconcile both of these observations, and in the process showa remarkable coincidence: decaying dark matter with MeV mass splittings canexplain both observations if positrons and photons are produced with similarbranching fractions. We illustrate this idea with an unstable branon, which isa standard WIMP dark matter candidate appearing in brane world models withlarge extra dimensions. We show that because branons decay via three-body finalstates, they are additionally unconstrained by searches for Galactic MeVgamma-ray lines. As a result, such unstable long-lifetime dark matter particlesprovide novel and distinct signatures that can be tested by future observationsof MeV gamma-rays.

Diffuse MeV Gamma-rays and Galactic 511 keV Line from Decaying WIMP Dark Matter