  Cosmic microwave background (CMB) constraints on dark matter annihilation are
a uniquely powerful tool in the quest to understand the nature of dark matter.
Annihilation of dark matter to Standard Model particles between recombination
and reionization heats baryons, ionizes neutral hydrogen, and alters the CMB
visibility function. Surprisingly, CMB bounds on dark matter annihilation are
not expected to improve significantly with the dramatic improvements in
sensitivity expected in future cosmological surveys. In this paper, we will
present a simple physical description of the origin of the CMB constraints and
explain why they are nearly saturated by current observations. The essential
feature is that dark matter annihilation primarily affects the ionization
fraction which can only increase substantially at times when the universe was
neutral. The resulting change to the CMB occurs on large angular scales and
leads to a phenomenology similar to that of the optical depth to reionization.
We will demonstrate this impact on the CMB both analytically and numerically.
Finally, we will discuss the additional impact that changing the ionization
fraction has on large scale structure.
