From our understanding of the universe as we see it we can safely say that ordinary matter all things we know are made of can account for only 5 percent of the total mass of the universe. The rest of 95 percent is made of dark energy and dark matter, elusive forms of energy and matter that haven't been yet directly observed. Furthermore, of that 5 percent of ordinary matter more than half is unaccounted for, meaning it should be around us although we cannot see it.

According to models, this missing matter is spread into interstellar and intergalactic space in the form of low density gas, connecting stars or galaxies inside clusters like a spider's web. Cosmic filaments, as they are called, are extremely hard to find especially at relatively low temperatures. However, at high temperature the gas starts to become visible in the X-ray wavelengths of the electromagnetic spectrum.

Previously futile efforts to find these cosmic filaments to study the evolution of cosmic webs have now materialized with a recent discovery made with ESA's XMM-Newton X-ray Space Observatory, revealing much of the missing ordinary matter in the universe.

The filament in question stretches between two galaxy clusters known as Abell 222 and Abell 223, located about 2.3 billion light years away from Earth. "The hot gas that we see in this bridge of filament is probably the hottest and densest part of the diffuse gas in the cosmic web, believed to constitute about half the baryonic matter in the universe", says Norbert Werner, research team leader at the SRON Netherlands Institute for Space Research.

Baryonic matter basically represents ordinary matter, or subatomic particles that make the matter as we know it, electrons, protons, neutrons and so on. "The discovery of the warmest of the missing baryons is important. That's because various models exist and they all predict that the missing baryons are some form of warm gas, but the models tend to disagree about the extremes", said Alexis Finoguenov.

Although the XMM-Newton telescope is extremely sensitive, the discovery was out of pure luck because the filament stretches in a straight line, concentrating the X-ray emission in a very small volume of space.

"This is only the beginning. To understand the distribution of the matter within the cosmic web, we have to see more systems like this one. And ultimately launch a dedicated space observatory to observe the cosmic web with a much higher sensitivity than possible with current missions. Our result allows to set up reliable requirements for those new missions", Werner explains.

"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. I expect many follow-up studies with XMM-Newton in the future targeting such highly promising regions in the sky", says XMM-Newton Project Scientist, Norbert Schartel.