Found!

You may have heard that the universe is made up of some different kinds of things. There's the very-likely-to-be-found dark matter, the something's-got-to-be-there-but-we-don't-know-what-for-sure of dark energy, and ordinary matter.

Dark matter and dark energy are postulated because the universe acts like it has a lot more mass than we see when we look at it. But until just this summer, even some of the ordinary matter hadn't been detected. We could only see about two-thirds of the ordinary or "baryonic" matter that should be there. This bugged scientists -- the whole idea behind dark matter and dark energy is that they're hard to find. The "dark" in their name refers to the way they're not supposed to interact with any other particles except through gravity. They don't reflect light or other radiation; they don't emit any radiation, and so on. So scientists had made their peace with the idea that those two things might require a lot of work to track down.

But baryonic matter was what the visible universe was made of. Any scientist who wanted to cast his or her eyes on a sample of it just had to look in a mirror -- and be doubly reassured, since both the mirror and the person in it were made up of baryonic matter. The fact that a third of it hadn't been accounted for was a wee bit galling.

However, several astronomers looking at quasars noticed that their view would sometimes be obscured, and eventually discovered massive filaments of superheated oxygen. Some more study and experimenting uncovered that these filaments of Warm-Hot-Intergalactic-Medium (WHIM) added up to the missing third of baryonic matter, likely solving this particular mystery.

Now just where the other 96 percent of the universe is, we've still got to figure out.