The SPI spectrometer aboard of the INTEGRAL satellitehas released a map of the e+e− annihilation emission lineof unprecedented quality, showing that most of the photons arise from aregion coinciding with the stellar bulge of the Milky Way. Theimpressive intensity (≃ 10-3 photon cm-2 s-1) andmorphology (round and wide) of the emission is begging an explanation. Different classes of astrophysical objects could injectpositrons in the interstellar medium of the bulge, but the onlyacceptable ones should inject them at energies low enough to avoidexcessive bremsstrahlung emission in the soft gamma ray regime. Amongthe ~ MeV injectors, none seems generous enough to sustain the highlevel of annihilation observed. Even the most profuse candidate, namelythe β+ radioactivity of 56Co nuclei created and expelled inthe interstellar medium by explosive nucleosynthesis of type Iasupernovae, falls short explaining the phenomenon due to the smallfraction of positrons leaking out from the ejecta (≈3%),together with the low SNIa rate in the bulge (≈0.03 percentury). It is therefore worth exploring alternative solutions, as for instance,the idea that the source of the positrons is the annihilation of lightdark matter (LDM) particles of the kind recently proposed, totally independently, by Bœhm and Fayet. Assuming that LDM is the culprit, crucialconstraints on the characteristics (mass and annihilation cross-section)of the associated particle may be discussed, combining direct gammaray 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 thegalactic bulge.
