  We calculate the fluxes of 511 KeV photons from the Galactic bulge caused by
positrons produced in the decays of relic particles with masses less than 100
MeV. In particular, we tighten the constraints on sterile neutrinos over a
large domain of the mass--mixing angle parameter space, where the resulting
photon flux would significantly exceed the experimental data. At the same time,
the observed photon fluxes can be easily caused by decaying sterile neutrinos
in the mass range 1 MeV < m_sterile < 50 MeV with the cosmological abundance
typically within 10^{-9} < Omega_sterile < 10^{-5}, assuming that Omega_sterile
comes entirely from the conversion of active neutrinos in the early Universe.
Other candidates for decaying relics such as neutral (pseudo)scalar particles
coupled to leptons with the gravitational strength can be compatible with the
photon flux, and can constitute the main component of cold dark matter.
