The vast majority of matter in the universe is dark—it is entirely invisible and detected only through its gravitational effects. Ordinary matter—everything from protons to planets to people—makes up only 16 percent. Unlike dark matter, ordinary matter emits light of various wavelengths and thus can be seen. But a large chunk of it is diffuse and spread thinly among halos that surround galaxies as well as in the vast spaces between galaxies.Due to its diffuse nature, roughly half of ordinary mat...