  If dark matter is made up of Weakly Interacting Massive Particles, the
annihilation of these particles in halos results in energy being released, some
of which is absorbed by gas, causing partial ionization and heating. Dark
matter annihilation may result in partial ionization and gas heating at high
redshifts, even before the formation of the first stars. It is shown that early
ionization results in a transfer of power to higher multipoles in the large
angle CMB polarization power spectra. Future CMB experiments may be able to
place constraints on certain light dark matter models. We also investigate the
effect of gas heating on the expected H21 cm power spectrum. Heating by
particle annihilation results in a decrease in the amplitude of the H21 cm
power spectrum as the gas temperature $T$ becomes comparable to the CMB
temperature $T_\gamma$, and then an increase as $T > T_\gamma$. The result is a
minimum in the power spectrum at the redshift for which $T \approx T_\gamma$.
Only certain models (low particle masses $\sim$ 10 GeV, or favorable halo
parameters) show this effect. Within these models, observations of the H21 cm
power spectrum at multiple redshifts can help us obtain constraints on dark
matter particle and halo properties.
