  It was recently proposed that stable particles of charge $-2$, $O^{--}$, can
exist and constitute dark matter after they bind with primordial helium in
O-helium (OHe) atoms. We study here in details the possibility that this model
provides an explanation for the excess of gamma radiation in the
positron-annihilation line from the galactic bulge observed by INTEGRAL. This
explanation assumes that OHe, excited to a 2s state through collisions in the
central part of the Galaxy, de-excites to its ground state via an E0
transition, emitting an electron-positron pair. The cross section for OHe
collisions with excitation to 2s level is calculated and it is shown that the
rate of such excitations in the galactic bulge strongly depends not only on the
mass of O-helium, which is determined by the mass of $O^{--}$, but also on the
density and velocity distribution of dark matter. Given the astrophysical
uncertainties on these distributions, this mechanism constrains the $O^{--}$
mass to lie in two possible regions. One of these is reachable in the
experimental searches for stable multicharged particles at the LHC.
