The Milky Way has existed for at least 13 billion years. Since then, it has continued to produce more and more new stars; one generation gives way to the next. To do this, it needs gas – more than it contains itself. That applies to all other galaxies too. But where do the Milky Way, Andromeda, and their like find more gas? In the intergalactic medium, which, at first glance, looks like just empty space between the galaxies.