A massive filament of hot gas connecting four galaxy clusters has been found by astronomers. The thread may hold part of the Universe’s “missing” matter, solving a long-standing enigma, given that it is ten times as big as the Milky Way. The study was published in Astronomy & Astrophysics.
The filament is made up of hot intergalactic gas (shown in mottled black-yellow), a type of ‘ordinary matter’ that has proven really difficult for astronomers to find. Image Credit: The European Space Agency
The finding was made by the scientists using the Suzaku X-ray space telescope from JAXA and the XMM-Newton instrument from the European Space Agency.
A significant portion of the visible components that form stars, planets, galaxies, and even life (more than one-third of the “normal” matter in the local universe) appears to be missing. Though this matter hasn’t been directly observed, it is essential for our models of the universe to remain consistent.
Current theories suggest this elusive matter might reside in long, thread-like structures of gas known as filaments, which stretch between the universe’s densest regions. While astronomers have detected these filaments before, studying them remains a challenge. They're typically very faint, making it difficult to separate their light from that of nearby galaxies, black holes, and other cosmic objects.
New research has now taken a significant step forward, successfully detecting and accurately characterizing a single filament of hot gas stretching between four galaxy clusters in the nearby universe.
For the first time, our results closely match what we see in our leading model of the cosmos – something that’s not happened before. It seems that the simulations were right all along.
Konstantinos Migkas, Study Lead Researcher, Leiden Observatory
XMM-Newton on the Case
The filament, which reaches temperatures of over 10 million degrees, contains approximately ten times the mass of the Milky Way and unites four galaxy clusters: two on one end and two on the other. All are members of the Shapley Supercluster, a grouping of around 8000 galaxies that make up one of the most enormous objects in the nearby universe.
The filament extends diagonally away from us through the supercluster for 23 million light-years, a distance comparable to traveling the Milky Way end to end around 230 times.
Konstantinos and colleagues defined the filament by combining X-ray observations from XMM-Newton and Suzaku with optical data from numerous additional sources.
The two X-ray telescopes made great partners. Suzaku tracked the filament's feeble X-ray radiation across a large area of space, while XMM-Newton highlighted contaminating X-ray sources - specifically, supermassive black holes - located inside the filament.
Thanks to XMM-Newton we could identify and remove these cosmic contaminants, so we knew we were looking at the gas in the filament and nothing else. Our approach was really successful, and reveals that the filament is exactly as we’d expect from our best large-scale simulations of the Universe.
Florian Pacaud, Study Co-Author, University of Bonn
Not Truly Missing
In addition to showing a massive and hitherto undiscovered thread of matter flowing across the local universe, the discovery demonstrates how some of the Universe’s densest and most extreme formations, galaxy clusters, are linked over vast distances.
It also sheds light on the nature of the ‘cosmic web’, the huge, unseen network of filaments that supports the structure of everything we see around us.
This research is a great example of collaboration between telescopes, and creates a new benchmark for how to spot the light coming from the faint filaments of the cosmic web.
Norbert Schartel, Project Scientist, ESA XMM-Newton
“More fundamentally, it reinforces our standard model of the cosmos and validates decades of simulations: it seems that the ‘missing’ matter may truly be lurking in hard-to-see threads woven across the Universe,” said Schartel.
The Euclid mission of the European Space Agency is in charge of assembling an accurate image of the cosmic web. Euclid, which was launched in 2023, investigates the structure and history of this web.
The mission will also delve deeply into the nature of dark matter and energy, both of which have never been detected despite accounting for 95% of the Universe, and collaborate with other dark Universe investigators to answer some of the most profound and long-standing cosmic puzzles.
Journal Reference:
Migkas, K., et al. (2025) Detection of pure warm-hot intergalactic medium emission from a 7.2 Mpc long filament in the Shapley supercluster using X-ray spectroscopy. Astronomy & Astrophysics. doi.org/10.1051/0004-6361/202554944.