The Cosmic Ionization History as a Probe of Dark Matter Decay and Annihilations

If the dark matter particles decay or annihilate, the resulting product particles could ionize the baryonic gas during the cosmic dark age. The ionization history of the Universe thus provides a powerful constraint on the decay and annihilation of dark matter. Recent CMB observations can be used to constrain the property of the dark matter. Here I review recent progresses on this constraint of dark matter property.