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 ^ { 8 } Be 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 .