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-1) 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 β+ radioactivity of 56Co 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 Bœhm 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.