The Warm, Hot Intergalactic Medium

Between the galaxies, there are these very long tendrils of gas between us and Andromeda. The acronym for it is WHIM, which you know, they didn't choose on a whim. It's just a whimsical name, you know, for something that's warm hot. This intergalactic plasma can actually be fairly hot on a temperature scale even though it's very, very dilute. And so that slows down photons that are moving through the universe from other galaxies.