The telescope detects radio waves that have been emitted by neutral hydrogen atoms. (newscientist.com)
Until that time, the Universe was composed almost entirely of neutral gas in the form of neutral hydrogen atoms. (skads-eu.org)
The shock wave then inflates and heats up the interstellar medium, which emits in the X-ray, and strips the electrons from surrounding neutral hydrogen atoms to make ionised hydrogen gas. (sciencedaily.com)