If much of the dark matter in the Universe consists of WIMPs, their annihilation releases energy, some of which ionizes the IGM. We calculate the contribution to the optical depth due to particle annihilation in early halos. This allows us to place bounds on the dark matter particle mass. We also consider the effect of halos on the H21 cm background. It is shown that larger halos (~ 10^6 solar masses) contain enough hot Hydrogen gas to produce a measurable H21 cm background. We present our conclusions.