  The SPI spectrometer aboard of the INTEGRAL satellite has released a map of
the e+e- annihilation emission line of unprecedented quality, showing that most
of the photons arise from a region coinciding with the stellar bulge of the
Milky Way. The impressive intensity (10^-3 photon/cm^2/s) and morphology (round
and wide) of the emission is begging an explanation. Different classes of
astrophysical objects could inject positrons in the interstellar medium of the
bulge, but the only acceptable ones should inject them at energies low enough
to avoid excessive bremsstrahlung emission in the soft gamma ray regime. Among
the ~ MeV injectors, none seems generous enough to sustain the high level of
annihilation observed. Even the most profuse candidate, namely the beta+
radioactivity of 56 Co nuclei created and expelled in the interstellar medium
by explosive nucleosynthesis of type Ia supernovae, falls short explaining the
phenomenon due to the small fraction of positrons leaking out from the ejecta
(3 %), together with the low SNIa rate in the bulge (0.03 per century).
  It is therefore worth exploring alternative solutions, as for instance, the
idea that the source of the positrons is the annihilation of light dark matter
(LDM) particles of the kind recently proposed, totally independently, by Boehm
and Fayet. Assuming that LDM is the culprit, crucial constraints on the
characteristics (mass and annihilation cross-section) of the associated
particle may be discussed, combining direct gamma ray observations and models
of the early Universe. In particular, the mass of the LDM particles should be
significantly less than 100 MeV, so that the e+ and e- resulting from their
annihilations do not radiate exceedingly through bremsstrahlung in the
interstellar gas of the galactic bulge.
