An article published in the journal “Nature” describes the discovery of the missing ordinary matter in the universe. A team of researchers led by Fabrizio Nicastro of INAF, Rome, Italy, discovered what is technically called baryonic matter after having searched it for almost twenty years using ESA’s XMM-Newton space telescope keeping under observation the quasar 1ES 1553+113 up to find the traces of the baryons hidden in the hot gas present among the galaxies.

The analysis of the Cosmic Microwave Background Radiation carried with increasing precision with the progress of the instruments used and culminated with the map created thanks to ESA’s Planck Surveyor satellite allowed to estimate the amount of the ordinary matter existing in the universe. Its evolution can be followed thanks to observations of the early universe possible with the new generation instruments but after the first two billion years of life of the universe over half of those baryons seemed to have disappeared.

Putting together all the matter that forms galaxies, including the halos that surround them, and even the gas that fills galactic clusters, you get only 20% of the expected ordinary matter. According to the theory developed over the years, most of the baryons are part of the filaments of gas that connect galaxies in a sort of immense cosmic web. This is mainly ionized hydrogen, whose emissions are very weak and therefore difficult to detect.

About twenty years ago, the era of X-ray space telescopes began to carry out high-resolution investigations also on mysteries such as that of the missing baryonic matter. However, that’s far from an easy research, in fact for many years the results have been disappointing. Between 2015 and 2017 Fabrizio Nicastro’s team managed to get the chance to conduct a series of observations of the quasar 1ES 1553+113 for a total of three weeks, a very long time that produced an enormous amount of data.

This quasar is about 4 billion light years away from Earth and is extremely bright thanks to a very active supermassive black hole because of a considerable amount of materials that orbit it and get heated considerably. The consequence is that it emits large amounts of electromagnetic radiation, including the X-rays detected by the XMM-Newton space telescope.

Those strong emissions are a sort of cosmic streetlight strong enough to show traces of two concentrations of oxygen present in the intergalactic gas between the quasar and the Earth, shown in the top image (ESA / ATG medialab; data: ESA / XMM-Newton / F. Nicastro et al 2018; cosmological simulation: R. Cen) by green and magenta arrows. Traces of nitrogen are indicated by the blue arrows.

That intergalactic gas is at temperatures estimated in part at hundreds of thousands of degrees and in part in millions of degrees and makes up the filaments of the cosmic web. Estimates of the various components of ordinary matter, together with those of dark matter and dark energy, are shown in the bottom image (ESA).

Fabrizio Nicastro and his colleagues plan to continue their studies using other quasars to find more intergalactic gas with XMM-Newton and with NASA’s Chandra space observatory to better understand the distribution of gas in cosmic filaments. The Athena (Advanced Telescope for High-Energy Astrophysics) mission will offer more precise measurements but the launch of this new ESA space telescope is scheduled for 2028.

Finding that intergalactic gas is already an excellent result, which rewards twenty years of efforts. It’s an important discovery that immediately dominated science news and beyond. It mentions missing matter but actually those baryons have always been in their place, it’s just that we couldn’t see them because of our instruments’ limitations. Now we have extended these limits a little farther away.