Galactic 511 keV line from MeV millicharged dark matter

We present a possible explanation of the recently observed 511 keV $\gamma$-ray anomaly with a new “millicharged” fermion. The new fermion is light [$\mathcal{O}(\mathrm{MeV})$] but has never been observed by any collider experiments mainly because of its tiny electromagnetic charge $\epsilon e$. We show that constraints from its relic density in the Universe and collider experiments allow a parameter range such that the 511 keV cosmic $\gamma$-ray emission from the galactic bulge may be due to positron production from this millicharged fermion.