  The Galactic positrons, as observed by their annihilation gamma-ray line at
0.511 MeV, are difficult to account for with astrophysical sources. It has been
proposed that they are produced instead by dark matter annihilation or decay in
the inner Galactic halo. To avoid other constraints, these processes are
required to occur "invisibly," such that the eventual positron annihilation is
the only detectable signal. However, electromagnetic radiative corrections to
these processes inevitably produce real gamma rays (``internal
bremsstrahlung"); this emission violates COMPTEL and EGRET constraints unless
the dark matter mass is less than about 20 MeV.
