Explanation of the 511 keV line: cascade annihilating dark matter with the 8Be anomaly

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(0)(d) pi(0)(d), and pi(0)(d) decays into e(+) e(-) with new interactions suggested by the Be-8 anomaly. Considering the constraints from the effective neutrino number N-eff and the 511 keV gamma-ray emission, a range of DM is obtained, 11.6 less than or similar to m(pi d)(+/-) less than or similar to 15 MeV. The typical DM annihilation cross section today is about 3.3 x 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.