If astronomers were in charge of a lost-and-found department, they would have been fired long ago. Most have come to terms with the notion that at least 90 percent of the matter in the universe must consist of some strange, dark material that they cannot directly detect. More embarrassingly, astronomers had lost track of most of the baryons—ordinary, visible matter made of protons, electrons, and neutrons.
In the standard picture of cosmic evolution, so-called dark matter provided the scaffolding upon which the baryons collected. During the first several billion years of cosmic history, the baryons—mostly hydrogen—formed vast gas clouds. Some ended up in galaxies and galaxy clusters, but that accounts for less than half the baryons