rAn orbital x-ray telescope has found a chunk of matter in the universe whose existence had long been theorised but evidence for which had been lacking, the European Space Agency (ESA) said on Wednesday. The discovery made by ESA's XMM-Newton telescope is part of so-called baryonic matter, which comprises less than five percent of the cosmos. Most of the universe consists of matter and energy of an unknown nature, which astrophysicists call "dark" and which is believed to be distributed in a web-like structure. "Dark energy," which causes an accelerated expansion of the universe after the Big Bang that created it, accounts for some 72 percent of the total, and "dark matter" -- heavy particles still waiting to be discovered -- accounts for around 23 percent, according to this theory. That leaves just 4.6 percent to comprise normal, or baryonic, matter, the category for the protons and neutrons that compose it. But only a small part of this stuff has been found. All the stars, galaxies and gas observed in the universe account for less than half of the baryons that should be there. The new claim is based on observation of a pair of distant galaxy clusters called Abell 222 and Abell 223 located 2.3 billion light years from Earth. Images and spectra found the two clusters were linked by a bridge of hot gas of a very low density. The astronomers believe that such low-density gas permeates the filaments of the cosmic web around the universe. They were able to spot this one because of its high temperature and because of a stroke of luck.
The thread was luckily in the telescope's line of sight, rather than visible from a narrower angle. "The hot gas that we see in this bridge or filament is probably the hottest and densest part of the diffuse gas in the cosmic web, which is believed to constitute about half of the baryonic matter in the universe," said lead researcher Norbert Werner of the SRON Netherlands Institute for Space Research.