Most of the gravitating matter in the universe has thus far gone undetected other than by its influence in shaping galaxies and aggregates of galaxies.
To account for the missing mass, various theories have been proposed, ranging from novel particles, including WIMPS (weakly interacting, and thus hard to detect, massive particles) and tiny black holes, which being black are hard to see against the void of outer space.
A more prosaic explanation is that there is just lots of cold dark hydrogen out there, which being cold, emits no radiation and is thus invisible.
Now astronomers Sebastiano Cantalupo and Xavier Prochaska and others at the Keck Observatory in Hawaii have observed the glow of a gigantic hydrogen cloud, ten times the diameter of our galaxy, which is illuminated by intense short-wave radiation from an ancient quasar.
How far this finding goes to account for the universe’s “missing matter,” no one seems to be saying. But it’s a cool discovery, even if the hydrogen observed had to be heated a little in order to be seen.