Abstract
A possible dark matter (DM) explanation about the long-standing issue of the Galactic 511 keV line is explored in this paper. For DM cascade annihilations of concern, a DM pair pi _d^{+} pi _d^{-} annihilates into unstable pi _d^{0} pi _d^{0}, and pi _d^{0} decays into e^+ e^- with new interactions suggested by the ^8Be anomaly. Considering the constraints from the effective neutrino number N_{mathrm {eff}} and the 511 keV gamma-ray emission, a range of DM is obtained, 11.6 lesssim m_{pi _d^{pm }} lesssim 15 MeV. The typical DM annihilation cross section today is about 3.3 times 10^{-29} cm^3 s^{-1}, which can give an explanation about the 511 keV line. The MeV scale DM may be searched by the DM-electron scattering, and the upper limit set by the CMB s-wave annihilation is derived in DM direct detections.
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