Ordinary matter—the kind composing stars, planets, and people—accounts for merely five percent of the universe, dwarfed by dark matter’s and dark energy's portions at 25 and 70 percent, respectively. Yet until recently, around 40 percent of that tiny matter budget was, in fact, unaccounted for. A new study has now identified the missing matter as hot gas lacing the spaces between the galaxies. Its discovery will help in further refining models of cosmic evolution. See also: Cosmology; Dark energy; Dark matter; Galaxy; Matter (physics); Planet; Star; Universe
In estimating the universe's inventory, scientists draw on multiple sources including the cosmic microwave background, a relic radiation of the big bang. The visible and otherwise inferable, ordinary matter contained in galaxies in the form of baryons—protons, neutrons, and the like—had, however, fallen significantly short of the estimated total. Researchers figured that the best bet for the missing matter was in the so-called cosmic web—the strands of matter associated with dark-matter filaments that extend between the galaxies. But this matter is so diffuse that it has eluded observation since the 1990s. See also: Baryon; Big bang theory; Cosmic background radiation; Neutron; Proton
The newfound success in detecting the matter came via the European Space Agency's (ESA) X-ray Multi-Mirror Mission (XMM-Newton) spacecraft. The observatory stared at a distant, bright galaxy known as a quasar. The observation time totaled 18 days, the longest observation conducted in x-rays of this kind of cosmic object. Over this vigil, XMM-Newton detected the weak x-ray signatures imparted to the quasar's light along the line of sight by oxygen gas. Calculating the density and quantity of material likely associated with the oxygen gas, and then extrapolating that figure to the cosmos as a whole, broadly matched the profile of the missing baryonic matter. Future spacecraft with higher sensitivities will seek to build on the findings, furthering the case that, at long last, the missing matter has been found. See also: Oxygen; Quasar; X-ray astronomy; X-rays
