84% of the matter in the universe is "dark", presently invisible to us except through its gravitational interactions. However, even tiny interactions between dark and visible matter could have striking effects on the primordial history of our universe, leaving traces of their existence in ionization and heating of the intergalactic gas, and in low-energy photons emitted during the cosmic dark ages and the epoch of reionization. I will explain how to estimate these effects, describe new and improved tools to predict the resulting signals in detail, and discuss current constraints and plans for future experimental tests.