Newfound clouds of missing ‘ordinary’ matter sit outside galaxies

For years, scientists have been calculating how much matter should exist in our universe. But more than half of it didn’t appear to be showing up in telescope images. Now an explanation for why is emerging. The “missing” matter is just too diffuse — or spread out — to see. Some of it may not even be made of complete atoms.

“The missing matter exists as a thin gas that’s very spread out,” says Haochen Wang. Wang works at the Massachusetts Institute of Technology in Cambridge. A PhD student in astrophysics, he led the new study. Think of the missing matter, he says, as faint clouds. They’re huge, stretching far beyond the edges of galaxies.

This missing matter is considered ordinary. By that, scientists mean it’s made of everyday particles called baryons. Those include the protons and neutrons that make up stars, planets and people. Ordinary matter is quite different from another type of unseen stuff ...