  We consider the 21cm absorption signal expected at high redshift in
cosmologies with and without non-baryonic 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 $\sim 2$ 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.
