This image shows the new filament, which connects four galaxy clusters: two on one end, two on the other. These clusters are visible as bright spots at the bottom and top of the filament (four white dots encircled by colour). A mottled band of purple stretches between these bright dots, standing out brightly against the black surrounding sky; this is the filament of X-ray-emitting hot gas that had not been seen before, and contains a chunk of ‘missing’ matter. (Credit: ESA/XMM-Newton and ISAS/JAXA)
In a nutshell
- Scientists detected missing ordinary matter in a 23-million-light-year cosmic filament between galaxy clusters using X-ray telescopes
- The discovery found gas heated to 10+ million degrees with density 100,000 times thinner than Earth’s atmosphere, solving part of the “missing baryon problem”
- This marks the first detection of such pristine, low-density cosmic gas, confirming computer simulations of how the universe’s largest structures are connected
LEIDEN, Netherlands — For decades, astronomers have been hunting for roughly 40% of the universe’s ordinary matter that seemed to have vanished without a trace. Now, researchers using powerful X-ray telescopes have found some of it hiding in a massive cosmic filament stretching 23.5 million light-years between galaxy clusters. It’s the first time scientists have detected this elusive material in such pristine conditions.
Scientists know exactly how much ordinary matter (the makeup of everything from your morning coffee to distant stars) should exist in the universe based on what the Big Bang created. But when they counted up all the matter they could see in stars, galaxies, and gas clouds, they came up short. About 40% of the universe’s ordinary matter had seemingly vanished.
The international team of astronomers, led by Konstantinos Migkas of the Leiden Observatory in the Netherlands, used a combination of X-ray telescopes to peer into what they call the warm-hot intergalactic medium, or WHIM. This is essentially a cosmic fog of extremely hot gas with temperatures reaching millions of degrees, hot enough to strip electrons from atoms but still too cool and spread out to easily detect.
How Scientists Located the Hidden Matter
Using data from Japan’s Suzaku X-ray telescope and the European Space Agency’s XMM-Newton observatory, the team studied a specific region in the Shapley supercluster, one of the largest known structures in the nearby universe. Galaxy clusters don’t just float randomly in space. They’re connected by vast bridges of matter called cosmic filaments, creating what astronomers call the “cosmic web.”
The filament they studied stretches between two pairs of galaxy clusters, designated A3530/32 and A3528-N/S. These clusters contain hundreds of galaxies each, but the real action happens in the nearly empty space between them. The researchers had to carefully account for X-ray sources from the galaxy clusters themselves and remove the glow from distant black holes to isolate the faint signal from the filament.
Their discovery revealed gas with temperatures around 0.8 to 1.1 kiloelectron volts (roughly 10 million degrees Fahrenheit), with a density about 100,000 times lower than Earth’s atmosphere. Despite being incredibly thin, the sheer volume of this cosmic highway means it contains substantial amounts of matter.
A Breakthrough in Cosmic Detection
Previous studies had detected similar cosmic filaments, but this marks the first time scientists have made such detailed measurements of a relatively uncontaminated example. Earlier discoveries often struggled to separate the filament’s signal from other sources like unresolved galaxies or active black holes scattered throughout the region.
“We report the direct imaging and spectroscopic detection of extended thermal WHIM emission from this single filament,” the researchers wrote in their study published in Astronomy & Astrophysics.
To determine the filament’s three-dimensional structure, the team combined their X-ray observations with optical data showing the distribution of galaxies along the same path. They found the filament tilts at a 53-degree angle relative to our line of sight, giving it a true length of 23.5 million light-years, or roughly 230 times the diameter of our Milky Way galaxy.
The team calculated that this single filament contains what they call a “baryon overdensity” of 30 to 40 times the average density of ordinary matter in the universe. Their imaging analysis revealed 21% more X-ray emission throughout the filament compared to the background sky, detected at a confidence level of 6.1 standard deviations, well above the threshold scientists typically require to claim a discovery.
“For the first time, our results closely match what we see in our leading model of the cosmos – something that’s not happened before,” says Migkas. “It seems that the simulations were right all along.”
The purple band comprises data from Suzaku. The astronomers were able to identify and remove any possible ‘contaminating’ sources of X-rays from the filament using XMM-Newton, leaving behind a pure thread of ‘missing’ matter. These sources can be seen here as bright dots studded through – and removed from – the filament’s emission. (Credit: ESA/XMM-Newton and ISAS/JAXA)
The Broader Impact on Our Understanding of the Universe
Understanding where ordinary matter resides helps scientists test their models of how the universe evolved over billions of years. The warm-hot intergalactic medium likely plays a crucial role in galaxy formation and evolution, serving as both a source of fresh material for star formation and a repository for gas ejected by galactic winds.
Future research will need to confirm whether other cosmic filaments show similar properties and determine how much of the universe’s missing ordinary matter resides in these structures.
“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,” says Norbert Schartel, an ESA XMM-Newton Project Scientist. “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.”
With new, more sensitive X-ray telescopes planned for the coming decade, scientists expect to map the cosmic web in unprecedented detail.
Paper Summary
Methodology
The research team used four observations from Japan’s Suzaku X-ray telescope combined with eight pointings from the European Space Agency’s XMM-Newton observatory to study a cosmic filament in the Shapley supercluster. They analyzed both surface brightness (imaging) and spectroscopic data in the X-ray portion of the electromagnetic spectrum, specifically the 0.5-7 keV energy range. To isolate the filament’s signal, researchers carefully subtracted contamination from nearby galaxy clusters and masked point sources like distant black holes. They used XMM-Newton’s higher angular resolution to identify contaminating sources that Suzaku couldn’t detect, then modeled these sources’ contributions to the Suzaku data. The team also analyzed optical spectroscopic data of galaxies to determine the filament’s three-dimensional geometry and confirm its existence as a coherent structure.
Results
The study detected a 7.2 million light-year cosmic filament connecting galaxy cluster pairs A3530/32 and A3528-N/S at a confidence level exceeding 6 standard deviations. Imaging analysis revealed 21% excess X-ray emission compared to background levels, while spectroscopic analysis measured gas temperatures of 0.8-1.1 keV (roughly 10 million degrees) and electron densities around 10^-5 particles per cubic centimeter. The filament shows a baryon overdensity of 30-40 times the cosmic average, representing some of the universe’s missing ordinary matter. The research confirmed the filament’s properties match predictions from cosmological simulations for the first time in an individual cosmic filament.
Limitations
The study relies on combining data from two different X-ray telescopes with different calibrations and instrumental backgrounds, potentially introducing systematic uncertainties. The analysis assumes certain parameters like gas metallicity and requires extrapolation of galaxy cluster properties beyond measured radii. Point source contamination, while carefully modeled, remains a potential source of bias. The team studied only one filament, so results may not be representative of cosmic filaments generally. Time-variable sources could affect measurements since the different telescope observations were taken years apart.
Funding and Disclosures
The research was supported by various European funding agencies including the German Federal Ministry for Economic Affairs and Energy, the European Union’s Horizon 2020 research program, the French Agence Nationale de la Recherche, and the Academy of Finland. The study used observations from space-based X-ray telescopes operated by international space agencies and publicly available optical survey data. No conflicts of interest were reported.
Publication Information
“Detection of pure warm-hot intergalactic medium emission from a 7.2 Mpc long filament in the Shapley supercluster using X-ray spectroscopy” by K. Migkas, F. Pacaud, T. Tuominen, and N. Aghanim was published in Astronomy & Astrophysics, volume 698, article A270 in 2025. The paper was received April 1, 2025, and accepted May 13, 2025.
