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. 
Ji-Haeng Huh, Jihn E. Kim, Jong-Chul Park, and Seong Chan Park