Most of the ordinary matter in the universe is hydrogen. But surprisingly, less than 20% of this hydrogen sits inside galaxies. The rest lies in the vast spaces between them – the so-called intergalactic medium.
This cosmic reservoir is thought to fuel the birth of new stars, as gas slowly falls into galaxies over billions of years. Yet much of that material doesn’t stay put: supernova explosions and powerful outflows from supermassive black holes can fling gas back out into intergalactic space.
