Féeton ($B-L$ Gauge Boson) Dark Matter for the 511-keV Gamma-Ray Excess and the Prediction of Low-energy Neutrino Flux

The féeton is the gauge boson of the $U(1{)}_{B-L}$ gauge theory. If the gauge coupling constant is extremely small, it becomes a candidate for dark matter. We show that its decay to a pair of electron and positron explains the observed Galactic 511-keV gamma-ray excess in a consistent manner. This féeton dark matter decays mainly into pairs of neutrino and anti-neutrino. Future low-energy experiments with improved directional capability make it possible to capture those neutrino signals. The seesaw-motivated parameter space predicts a relatively short féeton lifetime comparable to the current cosmological constraint.