Missing matter: Composite optical and X-ray image of galaxy clusters Abell 222 and Abell 223. The cluster pair is connected by a filament permeated by hot X-ray emitting gas.

Credit: ESA/ XMM-Newton/ EPIC/ ESO (J. Dietrich)/ SRON (N. Werner)/ MPE (A. Finoguenov)

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PARIS: An orbital X-ray telescope has found a mysterious chunk of matter in the universe – the existence of which had long been theorised, despite a lack of evidence.

The discovery made by the European Space Agency's XMM-Newton telescope is part of so-called 'baryonic matter', which comprises less than 5% of the cosmos.

Most of the universe consists of matter and energy of an unknown nature, which astrophysicists call 'dark' and believe is distributed in a web-like structure.

Only half the baryons

Dark energy, which caused an accelerated expansion of the universe after the Big Bang that created it, accounts for some 72% of the total, and dark matter (heavy particles still waiting to be discovered) accounts for around 23%, according to this theory.

That leaves just 4.6% 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 revealed that the two clusters were linked by a bridge of hot gas of a very low density.

German and Dutch astronomers behind the find detail it in the journal Astronomy and Astrophysics Letters. They believe that more low-density gas permeates the filaments of the cosmic web around the universe.

They were able to spot this filament 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.

Hot and dense

"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 in Utrecht.

"This is only the beginning," he added. "To understand the distribution of the matter within the cosmic web, we have to see many more systems like this – and ultimately launch a dedicated space research laboratory with a much higher sensitivity than possible with the current satellites," he added.

"This important breakthrough is great news for the mission. The gas has been detected after hard work and more importantly, we now know where to look for it," commented XMM-Newton project scientist, Norbert Schartel with the ESA. "I expect many follow-up studies with XMM-Newton in the future targeting such highly promising regions in the sky."