  We show that dark matter direct detection experiments are sensitive to the
existence of particles with a small effective charge (for instance, via
couplings to a kinetically mixed, low-mass dark photon). Our forecasts do not
depend on these particles comprising a significant fraction of the dark matter.
Rather, these experiments are sensitive to the irreducible abundance produced
in the early universe through the freeze-in mechanism. We find that ongoing and
proposed direct detection experiments will have world-leading sensitivity to
effective charges $Q\sim 10^{-12}$ across nine orders of magnitude in mass,
corresponding to a dark matter sub-fraction as low as $\sim 10^{-3}$.
