We consider the 21 cm absorption signal expected at high redshift in cosmologies with and without nonbaryonic cold dark matter. The expansion of the early Universe decelerates strongly with dark matter, but approximately coasts without it. This results in a different path length across the epochs when absorption is expected, with the consequence that the absorption is predicted to be a factor of ∼2 greater without dark matter than with it. Observation of such a signal would motivate consideration of extended theories of gravity in lieu of dark matter.