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 $N_{\mathrm{eff}}$ 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.
