Explanation of the 511 keV line: cascade annihilating dark matter with the ⁸ 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 π d⁺ π d⁻ annihilates into unstable π d⁰ π d⁰ , and π d⁰ decays into e⁺ e⁻ with new interactions suggested by the ⁸ Be anomaly. Considering the constraints from the effective neutrino number Neff and the 511 keV gamma-ray emission, a range of DM is obtained, 11.6 m_π d± 15 MeV. The typical DM annihilation cross section today is about 3.3 × 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.