  The 511 keV photons from the galactic center can be explained by positrons
produced through Q-ball decay. In the scheme of gauge-mediated supersymmetry
breaking, large Q balls with lepton charge are necessarily long-lived. In
particular, the lifetime can be as long as (or even longer than) the age of the
universe. If kinematically allowed, such large Q balls decay into positrons,
which eventually annihilate with electrons into 511 keV photons. Our scenario
is realized within the minimal supersymmetric standard model in the
inflationary universe, which is very plausible.
