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Top Comments: Filaments of Hot Matter Between Galaxy Clusters May Account for "Hidden" Matter [1]
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Date: 2025-06-22
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The current dominant theory of the universe (called Lambda-CDM) states that the universe contains about 5 % of ordinary matter, 27 % dark matter, and 68 % dark energy. If you’ve been reading my diaries, you know that physicists have been very actively searching for dark matter candidates, and that, at this point, nobody knows what it consists of. However, a less well-publicized problem is that astronomers have not yet found all of the ordinary matter expected to exist in the universe. What I mean by “ordinary matter” is the stuff we and the objects around us are made of, that is atoms. Physicists call is baryonic matter. Baryons are particles that respond to the strong nuclear force, and while there is a large zoo of particles in this category, the ones most common in ordinary matter are protons and neutrons, which make the greatest contribution to the mass of ordinary matter.
About 1/3 of the baryonic matter Lambda-CDM predicts to be present in the universe has not been observed. That doesn’t mean it’s missing—it’s likely in a form that is hard to observe. The challenge to astronomers, then, is to find evidence for this hidden matter. A recent study has found such evidence.
Cosmological models predict that the missing matter can be found in long filaments that extend between matter densities. Astronomers have spotted these filaments, but not with clarity. They're difficult to differentiate from the background because the matter exists as warm intergalactic gas called the warm-hot intergalactic medium (WHIM). The WHIM can reach extreme temperatures, up to 10 million degrees. At that temperature, the gas emits x-rays. But detecting it is not a simple matter of pointing an x-ray telescope at it because other things emit x-rays, too. In new research in Astronomy and Astrophysics, researchers explained how they found a filament of the missing matter by using two x-ray telescopes. It's titled "Detection of pure warm-hot intergalactic medium emission from a 7.2 Mpc long filament in the Shapley supercluster using X-ray spectroscopy." The lead author is Konstantinos Migkas of Leiden Observatory in the Netherlands.
By observing the x-rays emitted in the region of four galaxy clusters in the Shapely Supercluster, about 650 million lightyears away, with two different x-ray telescopes so as to be able to distinguish x-rays from the filaments from x-rays emitted by black holes and the like.
Image of four galaxy clusters, the warm-hot intergalactic medium (WHIM) between them, as well as contaminating x-ray sources.
The observed filament was determined to be 23.5 million lightyears long with a mass density about ten times larger than the Milky Way, our own galaxy. "Our findings agree well with the thermodynamic properties of filaments as predicted by cosmological simulations of the large-scale structure," the authors write.
These observations require confirmatory evidence from observations of other such filaments, but there’s a clear path to follow in order to account for the hidden baryonic mass of the universe.
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