Okay kiddo, so you know how space is really really big and mostly empty? Well, it's not completely empty. There are tiny bits of gas and dust floating around between galaxies.
This gas is called the intergalactic medium, or just IG for short. Sometimes, this gas can get really hot – like, millions of degrees hot! – and this is what we call the warm–hot intergalactic medium, or WHIM for short.
Scientists think that the WHIM is created when galaxies and galaxy clusters collide and merge. The crash can create shockwaves that heat up the gas in the IG, making it really hot.
Even though the WHIM is really hot, it's still really hard to see because it's so spread out. Scientists use special telescopes and other instruments to try and detect it.
Studying the WHIM can help us understand how galaxies and galaxy clusters form and evolve over time. Pretty cool, huh?