Dark matter annihilations or decays could generate high-energy particles which in turn would reshape the history of our cosmos, leaving traces of their existence in ionization and heating of the intergalactic medium, and in background radiation from the cosmic dark ages and the epoch of reionization. I will describe new and improved tools to map out the early-universe effects of exotic energy injections (including, but not limited to, dark matter annihilation and decay), and discuss current constraints and plans for future directions.