“This is the first real signal that stars are starting to form, and starting to affect the medium around them,” Alan Rogers, a scientist at MIT’s Haystack Observatory, said in a statement. “What’s happening in this period is that some of the radiation from the very first stars is starting to allow hydrogen to be seen. It’s causing hydrogen to start absorbing the background radiation, so you start seeing it in silhouette, at particular radio frequencies.”
https://www.cnet.com/news/astronomers-see-dark-matter-stars-signal-hydrogen-big-bang/