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Measurements of the temperature of the baryons at the end of the cosmic darkages can potentially set very precise constraints on energy injection fromexotic sources, such as annihilation or decay of the dark matter. However,additional effects that lower the gas temperature can substantially weaken theexpected constraints on exotic energy injection, whereas additional radiationbackgrounds can conceal the effect of an increased gas temperature inmeasurements of the 21-cm hyperfine transition of neutral hydrogen. Motivatedin part by recent claims of a detection of 21-cm absorption from a redshift of17 by the EDGES experiment, we derive the constraints on dark matterannihilation and decay that can be placed in the presence of extra radiationbackgrounds or effects that modify the gas temperature, such as darkmatter-baryon scattering and early baryon-photon decoupling. We find that ifthe EDGES observation is confirmed, then constraints on light dark matterdecaying or annihilating to electrons will in most scenarios be stronger thanexisting state-of-the-art limits from the cosmic microwave background,potentially by several orders of magnitude. More generally, our results allowmapping any future measurement of the global 21-cm signal into constraints ondark matter annihilation and decay, within the broad range of scenarios weconsider.

Too Hot, Too Cold or Just Right? Implications of a 21-cm Signal for Dark Matter Annihilation and Decay