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February 19th 2019

The Universe consists of dark matter and dark energy for respectively twenty-five and seventy percent. Only five percent is 'normal' matter-things that we can see, such as stars, planets and people. But even those five percent are difficult to detect. Estimates on the total amount of normal matter - called baryons - are based on the cosmic microwave background. This is the oldest light in the universe, originating from 380,000 years after the Big Bang.

Through observations of distant galaxies, astronomers are able to follow the evolution of baryonic matter during the first few billion years of the Universe. After that, almost half seems to be missing without a trace. First author Fabrizio Nicastro (Istituto Nazionale di Astrofisica (INAF), Italy) calls these missing baryons one of the greatest mysteries in modern astrophysics. "We know this matter must be out there, we see it in the early Universe, but then we can no longer get hold of it. Where did it go?"

If you add up all the stars and galaxies in the Universe, including the interstellar gas, you get about ten percent of all normal matter. If you add the hot diffuse gas in the halos around galaxies, plus the even hotter gas in clusters of galaxies, you are stuck with less than twenty percent. That is actually what you would expect, because stars, galaxies and clusters form in the densest knots of the cosmic web—the threadlike large-scale structure of the Universe - and those are rare.

Astronomers think that the 'missing' baryons are hiding in the filaments of the cosmic web, where matter is rarer and therefore more difficult to observe. Up to now, only sixty percent of this intergalactic matter has been localized.

In 2015 and 2017, Nicastro and his colleagues observed for a total of eighteen days with ESA's X-ray telescope XMM-Newton, looking at a quasar at four billion light-years away. Quasars are large galaxies with a supermassive black hole in the center and they shine brightly at X-ray and radio wavelengths. In the data, the researchers found the fingerprint of oxygen in the hot intergalactic gas between us and the distant quasar, at two locations along the line of sight. Second author Jelle Kaastra (SRON Netherlands Institute for Space Research): "Enormous stocks of matter are out there, including oxygen, in the quantities we expected. It seems that we can finally solve the mystery of the missing baryons."

The result is the beginning of a new quest. The astronomers will now start investigating new quasars with both XMM-Newton and NASA's Chandra observatory. Co-author Nastasha Wijers (PhD student at Leiden Observatory): "We now want to look at other sources in the Universe to confirm that our results are universal. We also want to look further into the newly found matter." Co-author Joop Schaye (Leiden Observatory) adds: "This is an exciting first step. We are also looking forward to the launch of Athena in 2031, which allows us to study the warm intergalactic medium in great detail because of its much greater sensitivity."