Astronomy & Astrophysics is publishing the discovery by Dutch and German astronomers  of a filament of tenuous hot gas connecting two clusters of galaxies. The existence of this hot gas (with a temperature of 100 000 - 10 000 000 degrees), known as a warm-hot intergalactic medium, was predicted 10 years ago as a possible source for the missing dark matter. Gas at such high temperature and low density is very difficult to detect and many attempts have failed in past years. The team observed a pair of clusters of galaxies (Abell 222 and Abell 223) using the European X-ray satellite XMM-Newton. Their observations (see Fig. 1) clearly show a bridge connecting both clusters. The gas they observed there is probably the hottest and densest part of the diffuse gas in the cosmic web, which would be part of the missing baryonic dark matter.
Most of the matter and energy in the Universe is of unknown nature, so they are called dark matter and dark energy. Dark energy accounts for 72% of the total energy in the Universe, while some 23% of the total amount of matter/energy is made of this so-called dark matter, which is composed of heavy particles still waiting to be discovered by particle physicists. The remaining 5% of the Universe is made of ordinary matter, the one we know on Earth that constitutes stars and planets. It consists of protons and neutrons called baryons and of electrons, all the building blocks of the atoms. But part of this 5% of baryonic matter is also missing. Stars, galaxies, and gas that astronomers observed in the Universe account for less than half of the baryonic matter.
The newly-detected bridge connecting Abell 222 and Abell 223 would be part of this missing baryonic matter. Matter in the Universe is distributed in a web-like structure, and clusters of galaxies are the dense nodes of this cosmic web . For 10 years, astronomers suspected that the missing baryonic matter is hot gas at very low density permeating the filamentary structure of the cosmic web. Because of its low density, detecting this hot gas was a very challenging task. This discovery was made possible because of the very fortunate geometry of the two clusters. As seen from the Earth, the filament connecting the two clusters is aligned along our line-of-sight, so that the entire emission from the filament is concentrated in a small region of the sky, thereby making its detection possible. Previous observations, at a lower sensitivity level, only allowed astronomers to detect the clusters and some groups of galaxies, the dense knots of the web. The high-sensitivity level now achieved with in-depth XMM-Newton observations makes it possible to observe the connecting wires of the cosmic web. This discovery is a step toward understanding the distribution of the matter within the large-scale structure of the Universe.