  The $B-L$ gauge symmetry motivated from the successful generation of the
seesaw mechanism and leptogenesis. We show that if the $B-L$ gauge boson
constitutes a small fraction of the dark matter (DM) it can explain the
Galactic $511$ keV emission via the decay into an electron-positron pair. We
find the model parameter space that is consistent with the seesaw mechanism,
cosmologically viable, and accounting for the amplitude of the Galactic
positron line. From this parameter space we derive an upper bound of the gauge
boson mass and then an bound of the positron injection energy $\lesssim3$ MeV.
This derived energy bound is consistent with the observational upper limit of
the injection energy. The resultant model predicts the $B-L$ breaking scale to
be in a relatively narrow range, i.e., $V_{B\textrm{--}L}\sim10^{15}
\textrm{--} 10^{16} $GeV, which is consistent with a Grand Unification (GUT)
scale seesaw mechanism. The model is consistent in several phenomenologies,
suggesting their common origin from the B-L symmetry breaking.
