Finding Ordinary Matter is No Ordinary Matter

Today, thanks to some very beautiful cosmology, we think we know quite a bit about the matter content of our universe. The observed universe is, according to these studies, 1 part ordinary matter, 5 parts dark matter, and 14 parts dark energy. One of the interesting gaps in our understanding of this picture, however, is that when we add up all the numbers, we find that we are missing between 30 to 40 percent of the ordinary matter. One possibility for where this matter may be found is in hot (10^6 K) low density gas in the intergalactic medium. At these high temperatures, atoms like oxygen and nitrogen retain a few bound electrons. But because these are heavy elements with a few bound electrons they will absorb only at very high energies. In order to see this absorbtion, you need to look in the ultraviolet or X-ray regime of the spectrum. Since it’s impossible to test this theory from ground-based instruments, this idea has floated around, but never really been verified.
Now there is news today, published in Nature by Nicastro et. a (vol. 433, p.493), that the Chandra space telescope has indeed detected evidence of this absorption and, with admittedly still large uncertainty, the calculations suggest that indeed this indeed makes the calculations for ordinary matter add up.