  Dark matter particles need not be completely stable, and in fact they may be
decaying now. We consider this possibility in the frameworks of universal extra
dimensions and supersymmetry with very late decays of WIMPs to Kaluza-Klein
gravitons and gravitinos. The diffuse photon background is a sensitive probe,
even for lifetimes far greater than the age of the Universe. Remarkably, both
the energy spectrum and flux of the observed MeV gamma ray excess may be
naturally explained by decaying dark matter with MeV mass splittings. Future
observations of continuum and line photon fluxes will test this explanation and
may provide novel constraints on cosmological parameters.
