XMM-Newton discovers part of the missing matter in the universe

Astronomy & Astrophysics is publishing the discovery by Dutch and German astronomers [1] 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.