A new constraint on millicharged dark matter from galaxy clusters
  We propose a new constraint on millicharged dark matter from considerations
on galaxy clusters. The charged dark matter moves under the influence of the
randomly oriented magnetic fields in galaxy clusters, and the corresponding
dark matter density profile can significantly differ from the concordance CDM
predictions which are well supported from the galaxy cluster observations. With
a typical amplitude of magnetic fields $B=\mathcal O(1)\,\mu$G and dark matter
velocity $v=\mathcal O(100)\,$km/sec at a cluster radius $R\simeq 1\,$Mpc, we
claim that the charge $\epsilon e$ ($e$ is the elementary charge) of dark
matter with mass $m$ should be bounded as $\epsilon \lesssim 10^{-14}(m/{\rm
GeV})$ which is substantially tighter than the other previous constraints.
