  We revisit the possibility of constraining the properties of dark matter (DM)
by studying the epoch of cosmic reionization. Previous studies have shown that
DM annihilation was unlikely to have provided a large fraction of the photons
that ionized the universe, but instead played a subdominant role relative to
stars and quasars. The DM, however, begins to efficiently annihilate with the
formation of primordial microhalos at $z\sim100-200$, much earlier than the
formation of the first stars. Therefore, if DM annihilation ionized the
universe at even the percent level over the interval $z \sim 20-100$, it can
leave a significant imprint on the global optical depth, $\tau$. Moreover, we
show that cosmic microwave background (CMB) polarization data and future 21 cm
measurements will enable us to more directly probe the DM contribution to the
optical depth. In order to compute the annihilation rate throughout the epoch
of reionization, we adopt the latest results from structure formation studies
and explore the impact of various free parameters on our results. We show that
future measurements could make it possible to place constraints on the dark
matter's annihilation cross section that are at a level comparable to those
obtained from the observations of dwarf galaxies, cosmic ray measurements, and
studies of recombination.
