The dark matter problem is famous: about 80% of all mass in the Universe is invisible to light. However, a lot of regular matter is also “missing”, in the sense that we know it exists, but haven’t determined where it’s located. Measurements of the cosmic microwave background have shown how many atoms there are in the Universe, but they aren’t in galaxies. A new observation using the Chandra X-ray Observatory may show that at least some of the missing atoms are in giant hot gas clouds surrounding galaxies.

If the atoms were uniformly distributed around the galaxy, then they would make a sphere over 600 thousand light-years across, about six times the diameter of the Milky Way’s disk. Since the atoms are almost certainly not in a uniform cloud, the actual size of the cloud can’t be known, so the researchers weren’t able to determine whether the oxygen was in the CGM or the IGM. Either way, it is consistent with theoretical predictions. [Read more….]