  If dark matter (DM) is millicharged or darkly charged, collective plasma
processes may dominate momentum exchange over direct particle collisions.
Plasma streaming instabilities can couple the momentum of the DM to
counter-streaming baryons or other DM and result in the counter-streaming
fluids coming to rest with each other, just as happens for baryonic
collisionless shocks in astrophysical systems. While electrostatic plasma
instabilities are highly suppressed by Landau damping when DM is millicharged,
in the cosmological situations of interest, electromagnetic instabilities such
as the Weibel can couple momenta, assuming that the linear instability
saturates in the manner typically found for baryonic plasmas. We find that the
streaming of DM in the pre-Recombination universe is affected more strongly by
direct collisions than collective processes, validating previous constraints.
However, when considering magnetized Weibel and Firehose instabilities, the
properties of the Bullet Cluster merger are likely to be substantially altered
if $[q_\chi/m_\chi] \gtrsim 10^{-12}-10^{-11}$, where $[q_\chi/m_\chi]$ is the
charge-to-mass ratio of the DM relative to that of the proton. The Weibel
growth rates are even faster in the case of a dark-$U(1)$ charge, potentially
ruling out $[q_\chi/m_\chi] \gtrsim 10^{-14}$ in the Bullet Cluster system, in
agreement with previous work. The strongest previous limits on millicharged DM
(mDM) arise from considering the spin-down of galactic disks. We show that
plasma instabilities or tangled background magnetic fields could lead to
diffusive propagation of the DM, weakening these spin-down limits. Thus, plasma
instabilities may place some of the most stringent constraints over much of the
millicharged, and our results corroborate previous extremely stringent
potential constraints on the dark-charged parameter space.
