The 21cm wavelength arises from changes within the atoms of hydrogen, the Universe's most abundant element, and one that can tell us much about the early Universe before heavier elements were formed. A key insight lies in the different speed limits for dark matter and normal matter in the early Universe, first pointed out in a 2010 Physical Review D paper. The early Universe was shaped in part by pressure waves - just like sound waves - created in the wake of the Big Bang. Like air molecules shifted around by sounds, these waves carried and distributed normal matter in regular patterns we can now observe. But dark matter, because it does not interact with normal matter, was not swayed by the waves, responding only to gravity.
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