If there are two things we know about the universe, it’s that it’s big and there’s a lot of it. So much in fact, that we didn’t know where half of it was until recently.
Now before you get your hopes up, I’m not talking about dark matter or dark energy, which combined make up over 95% of the universe. I know you’re all as antsy as I am to find out what dark matter and dark energy really are, but that mystery remains unsolved. In fact, I’m talking about plain old baryonic matter, like you and me and everything we know. The stuff that makes up less than 5% of the universe. It turns out, we couldn’t even track half of that stuff down.
That’s not to say we didn’t have a pretty good idea of where it was. The matter that we could account for was where you’d expect it to be, bundled up in two trillion galaxies each with an average of hundreds of billions of stars across the observable universe.
In between those galaxies we think are filaments of dark matter, linking them together in a mind-breakingly huge cosmic web. We suspected the gravity of that dark matter also attracted regular matter too, which had been turned into plasma after ultraviolet radiation from early stars separated atoms from their electrons. This plasma is what’s known as the WHIM, or Warm-Hot Intergalactic Medium.