Explanation of the 511 keV line: Cascade annihilating dark matter with the ${}^8$Be 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 ${\pi }_d^0{\pi }_d^0$, 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 $N_{\mathrm{e}\mathrm{f}\mathrm{f}}$ and the 511 keV gamma-ray emission, a range of DM is obtained, $11.6 \lesssim m_{\pi_d^{\pm}} \lesssim 15$MeV\mathrm{.}ThetypicalDMannihilationcrosssectiontodayisabout3.3$\times$ ${10}^{-29}$ 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.