We know how much total matter is in the universe, including "normal" matter (things like stars, planets, you, me) and more mysterious "dark" matter. However, only about 10% of the normal matter has been accounted for in surveys of galaxies. Astronomers have long suspected the missing matter sits in threads of gas between galaxies, but it's hard to detect because it's very tenuous.
What's new: Researchers now report spotting about 30% of the missing matter by looking for its shadow in the cosmic microwave background — the afterglow of the big bang itself. The background light filters through the gas and gets bumped up to higher energies through collisions with the hot particles, leading to a subtly higher observed temperature between galaxies.
Why it matters: While it doesn't completely solve the mystery of the missing matter, the research may help us paint a fuller family portrait of the universe. By understanding what it's made of, we can learn more about its past and future evolution.