Finding missing matter in the cosmic web

Galaxies, gas and dark matter © ESA/ESO/NASA

Galaxies, gas and dark matter © ESA/ESO/NASA

Astronomers from the University of Geneva and EPFL show that the universe’s missing “ordinary matter” is found in the form of a very hot gas in intergalactic filaments.

Image: Components of the galaxy cluster Abell 2744, also known as the Pandora Cluster: galaxies (white), hot gas (red) and dark matter (blue). Credits: ESA/XMM-Newton (X-rays); ESO/WFI (optical); NASA/ESA & CFHT (dark matter)

Most physical objects are made up of a type of matter known as “ordinary”. Nonetheless, ordinary matter corresponds to only 5% of the universe, but about half of this lies beyond our means of detection. Computer simulations predict that the unseen part of ordinary matter should be located in the “cosmic web”: galaxies and galaxy clusters linked in a gigantic filamentary network, with temperatures between 100,000 and 10 million degrees Celsius. The University of Geneva has now led a team of researchers including EPFL who show that most of the missing ordinary matter is found as a very hot gas associated with intergalactic filaments. The discovery is published in Nature.

Galaxies tend to group together and then congregate into galaxy clusters, the universe’s largest structures, held together by gravity. But galaxy clusters also contain large amounts of hot gas and even larger amounts of invisible dark matter. The distribution of ordinary matter in the Universe is not homogeneous. Instead, under the action of gravity, matter is concentrated into filamentary structures, forming a network of knots and links called the “cosmic web”.

Galaxies and galaxy clusters are located within a giant filamentary network, where the biggest clusters are located in the densest hubs. The cosmic web is mostly made up of dark matter and some ordinary matter; computer simulations indicate that it acts as the “scaffolding of the cosmos” – a framework where stars, galaxies and clusters can form and evolve.

Astrophysicists from UNIGE and EPFL (Laboratory of Astrophysics) have now discovered that the cosmic web contains the missing ordinary matter of the universe. The researchers observed a massive galaxy cluster called Abell 2744 using the European Space Agency’s (ESA) XMM-Newton X-ray space telescope, which can detect the signature of very hot gases by their X-ray signature.

Drawn by gravitational forces, matter is concentrated into the filamentary structures that make up the cosmic web. The areas that experience the highest gravitational force collapse to form the “knots” of the network. Abell 2744 is one such knot. Like in a neural network, these knots connect to one another through filaments.

Observing the filaments with the XMM-Newton telescope, the researchers detected the presence of gas. With that, they also found the missing part of ordinary matter, which the field refers to as “missing baryons”.

The astrophysicists pointed XMM in the direction of the areas where they suspected to find the presence of filaments, and therefore, the presence of hot gas structures at temperatures of 10 million degrees Celsius. For the first time ever, they were able to measure the temperature and density of these structures, and found that they corresponded to the predictions of previous computer simulations.

The discovery offers a significant validation of galaxy-formation models of in the Universe. “Now we must verify that the discovery of Abell 2744’s missing baryons is applicable to the entire universe,” says lead researcher Dominique Eckert. “This will consist of studying these filamentary regions in detail, and measuring their temperature distribution and the various atoms that compose them, in order to understand how many heavy elements there are in the universe.” If the researchers can measure the atoms in these filaments, they will be able to estimate the number of heavy nuclei formed by stars since the beginning of the universe.

The ESA is now developing a new space telescope to further pursue this line of research, in which Switzerland and the researchers from UNIGE are especially involved. The telescope, named Athena, is expected to be operational in the mid-2020s.

This work involved collaboration between UNIGE’s Department of Astronomy, EPFL’s Laboratory of Astrophysics (Observatoire de Sauverny), INAF - IASF Milano, the University of KwaZulu-Natal (Durban SA), Aix Marseille Université (CNRS-LAM), Argelander-Institut für Astronomie (Bonn), and the Lyon Observatory (Université Lyon).


Eckert D, Jauzac M, Shan HY, Kneib J-P, Erben T, Israel H, Jullo E, Klein M, Massey R, Richard J, Tchernin C. Warm–hot baryons comprise 5–10 per cent of filaments in the cosmic web. Nature 03 December 2015. DOI: 10.1038/nature16058