511 keV $gamma$-ray emission from the galactic bulge by MeV millicharged dark matter

We propose a possible explanation for the recently observed anomalous 511 keV line with a new millicharged fermion. This new fermion is light [${cal O}({rm MeV})$]. Nevertheless, it has never been observed by any collider experiments by virtue of its tiny electromagnetic charge $epsilon e$. In particular, we constrain parameters of this millicharged particle if the 511 keV cosmic $gamma$-ray emission from the galactic bulge is due to positron production from this new particle.