Although it makes up everything we see and touch on a daily basis, ordinary (or baryonic) matter is relatively rare in the universe, and weirder still there seems to be a huge chunk of it missing. After 20 years of scouring the sky, astronomers using the ESA's XMM-Newton observatory have now found clues to this missing matter hiding in intergalactic gas.

When you break down the contents of the universe, roughly 70 percent of it is a force known as dark energy, which is said to be accelerating the expansion of the universe. Another 25 percent or so is dark matter, that mysterious, invisible stuff that only seems to make itself known through gravity. The remaining five percent is ordinary matter, which includes us humans, the Earth, the Sun, and all the other stars and planets out there.

That last little bit is the focus here, and it's been surprisingly difficult to account for where that five percent is hiding. Since looking through space is essentially the same as looking back through time, observations of the most distant objects have shown how the matter budget has changed over billions of years, but after a certain point in history astronomers lose track of a huge amount of it.

"The missing baryons represent one of the biggest mysteries in modern astrophysics," says Fabrizio Nicastro, lead author of the study. "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?"

Adding up all the stars, planets, dust and gas we see in and around galaxies and galaxy clusters only gets us to about 18 percent of the ordinary matter we should expect to see. Looking at the cool and warm intergalactic gas, which connects the galaxy clusters in long filaments to form a "cosmic web", adds an extra 43 percent or so. But that still leaves almost 40 percent unaccounted for.

And now researchers on the new study say they've found signals indicating where that long-lost matter might be. Using ESA's XMM-Newton X-ray space observatory, the team observed a quasar that lies more than 4 billion light-years away, over a total of 18 days between 2015 and 2017. By studying how the X-ray signals from the quasar are obscured during their long journey, astronomers can determine what kind of matter lies between it and us. Sure enough, the researchers found what they were looking for.

"After combing through the data, we succeeded at finding the signature of oxygen in the hot intergalactic gas between us and the distant quasar, at two different locations along the line of sight," says Nicastro. "This is happening because there are huge reservoirs of material – including oxygen – lying there, and just in the amount we were expecting, so we finally can close the gap in the baryon budget of the universe."

To find out whether these readings occur everywhere or might be an isolated incident, the researcher plan to repeat the study with other quasars in the future, using XMM-Newton and NASA's Chandra X-ray Observatory. Confirmation would help improve our understanding of the structure and evolution of the universe.

The research was published in the journal Nature.

Source: ESA

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