Astronomers have discovered a vast tendril of hot gas linking four galaxy clusters and stretching out for 23 million light-years, 230 times the length of our galaxy. With 10 times the mass of the Milky Way, this filamentary structure accounts for much of the universe’s “missing matter,” the search for which has baffled scientists for decades.

This “missing matter” doesn’t refer to dark matter, the mysterious stuff that remains effectively invisible because it doesn’t interact with light (sadly, that remains an ongoing puzzle). Instead, it is “ordinary matter” made up of atoms, composed of electrons, protons, and neutrons (collectively called baryons) which make up stars, planets, moons, and our bodies.

For decades, our best models of the universe have suggested that a third of the baryonic matter that should be out there in the cosmos is missing. This discovery of that missing matter suggests our best models of the universe were right all along. It could also reveal more about the “Cosmic Web,” the vast structure along which entire galaxies grew and gathered during the earlier epochs of our 13.8 billion-year-old universe.

The aforementioned models of the cosmos, including the standard model of cosmology, have long posited the idea that the missing baryonic matter of the universe is locked up in vast filaments of gas stretching between the densest pockets of space.

Though astronomers have seen these filaments before, the fact that they are faint has meant that their light has been washed out by other sources like galaxies and supermassive black hole-powered quasars. That means the characteristics of these filaments have remained elusive.

But now, a team of astronomers has for the first time been able to determine the properties of one of these filaments, which links four galactic clusters in the local universe. These four clusters are all part of the Shapley Supercluster, a gathering of over 8,000 galaxies forming one of the most massive structures in the nearby cosmos.

“For the first time, our results closely match what we see in our leading model of the cosmos – something that’s not happened before,” team leader Konstantinos Migkas of Leiden Observatory in the Netherlands said in a statement. “It seems that the simulations were right all along.”

Missing matter is hot stuff

The newly observed filament isn’t just extraordinary in terms of its mass and size; it also has a temperature of a staggering 18 million degrees Fahrenheit (10 million degrees Celsius). That’s around 1,800 times hotter than the surface of the sun.

The filament stretches diagonally through the Shapely Supercluster.

Vital to the characterization of this filament was X-ray data from XMM-Newton and Suzaku, which made a great tag-team of telescopes.

While Suzaku, a Japan Aerospace Exploration Agency (JAXA) satellite, mapped X-ray light over a vast region of space, the European Space Agency (ESA) operated XMM-Newton zoomed in of X-ray points from supermassive black holes studded within the filament, “contaminating” it.

“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,” team member and University of Bonn researcher Florian Pacaud said. “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.”

The team then combined these X-ray observations with optical data from a plethora of other telescopes.

Revealing this hitherto undiscovered tendril of hot matter connecting galaxy clusters has the potential to aid scientists’ understanding of these extreme structures and how they are connected across vast cosmic distances.

This could, in turn, aid our understanding of the Cosmic Web, filaments of matter that acted as a cosmic scaffold helping the universe to assemble in its current form.

“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,” XMM-Newton Project Scientist Norbert Schartel explained. “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.”

The team’s research was published on Thursday (June 19) in the journal Astronomy & Astrophysics.

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Astronomers have discovered a huge filament of hot gas bridging four galaxy clusters. At 10 times as massive as our galaxy, the thread could contain some of the Universe’s ‘missing’ matter, addressing a decades-long mystery.

The astronomers used the European Space Agency’s XMM-Newton and JAXA’s Suzaku X-ray space telescopes to make the discovery.

Over one-third of the ‘normal’ matter in the local Universe – the visible stuff making up stars, planets, galaxies, life – is missing. It hasn’t yet been seen, but it’s needed to make our models of the cosmos work properly.

Said models suggest that this elusive matter might exist in long strings of gas, or filaments, bridging the densest pockets of space. While we’ve spotted filaments before, it’s tricky to make out their properties; they’re typically faint, making it difficult to isolate their light from that of any galaxies, black holes, and other objects lying nearby.

New research is now one ofthe first to do just this, finding and accurately characterizing a single filament of hot gas stretching between four clusters of galaxies 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,” says lead researcher Konstantinos Migkas of Leiden Observatory in the Netherlands. “It seems that the simulations were right all along.”

XMM-Newton on the case

Clocking in at over 10 million degrees, the filament contains around 10 times the mass of the Milky Way and connects four galaxy clusters: two on one end, two on the other. All are part of the Shapley Supercluster, a collection of more than 8000 galaxies that forms one of the most massive structures in the nearby Universe.

The filament stretches diagonally away from us through the supercluster for 23 million light-years, the equivalent of traversing the Milky Way end to end around 230 times.

Konstantinos and colleagues characterized the filament by combining X-ray observations from XMM-Newton and Suzaku, and digging into optical data from several others.

The two X-ray telescopes were ideal partners. Suzaku mapped the filament’s faint X-ray light over a wide region of space, while XMM-Newton pinpointed very precisely contaminating sources of X-rays – namely, supermassive black holes – lying within 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,” adds co-author Florian Pacaud of the University of Bonn, Germany. “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.”

Not truly missing

As well as revealing a huge and previously unseen thread of matter running through the nearby cosmos, the finding shows how some of the densest and most extreme structures in the Universe – galaxy clusters – are connected over colossal distances.

It also sheds light on the very nature of the ‘cosmic web’, the vast, invisible cobweb of filaments that underpins 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,” adds Norbert Schartel, 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.”

Piecing together an accurate picture of the cosmic web is the domain of ESA’s Euclid mission. Launched in 2023, Euclid is exploring this web’s structure and history. The mission is also digging deep into the nature of dark matter and energy – neither of which have ever been observed, despite accounting for a whopping 95% of the Universe – and working with other dark Universe detectives to solve some of the biggest and longest-standing cosmic mysteries.

Evidence continues to assemble that dwarf galaxies played a larger role in shaping the early universe than previously thought.

Astronomers analyzing data from the James Webb Space Telescope (JWST) have uncovered a population of tiny, energetic galaxies that may have been key players in clearing the cosmic fog that shrouded the universe after the Big Bang.

“You don’t necessarily need to look for more exotic features,” Isak Wold, an assistant research scientist at the Catholic University of America in Washington D.C., told reporters during the 246th meeting of the American Astronomical Society in Alaska. “These tiny but numerous galaxies could produce all the light needed for reionization.”

About 380,000 years after the Big Bang, the universe cooled enough for charged particles to combine into neutral hydrogen atoms, creating a thick, light-absorbing fog, an era known as the cosmic dark ages. It wasn’t until several hundred million years later, with the birth of the first stars and galaxies, that intense ultraviolet (UV) radiation began reionizing this primordial hydrogen. That process gradually cleared the dense fog, allowing starlight to travel freely through space and illuminating the cosmos for the first time.

For decades, astronomers have debated what triggered this dramatic transformation. The leading candidates included massive galaxies, quasars powered by black holes, and small, low-mass galaxies. New data from the JWST now points strongly to the smallest contenders, suggesting these tiny galaxies acted like cosmic flashlights lighting up the early universe.

To identify these early galaxies, Wold and his colleagues focused on a massive galaxy cluster called Abell 2744, or Pandora’s Cluster, located about 4 billion light-years away in the constellation Sculptor. The immense gravity of this cluster acts as a natural magnifying glass, bending and amplifying light coming from much more distant, ancient galaxies behind it. Tapping into this quirk of nature, combined with the JWST’s powerful instruments, the researchers peered nearly 13 billion years back in time.

A gif showing the identification of the dwarf galaxies in Pandora’s cluster (Image credit: NASA)

Using the JWST’s Near-Infrared Camera (NIRCam) and Near-Infrared Spectrograph (NIRSpec), the team searched for a specific green emission line from doubly ionized oxygen, a hallmark of intense star formation. This light was originally emitted in the visible range but was stretched into the infrared as it traveled through the expanding universe, according to a NASA statement.

The search yielded 83 tiny, starburst galaxies, all vigorously forming stars when the universe was just 800 million years old, around 6% of its current age.

“Our analysis […] shows they existed in sufficient numbers and packed enough ultraviolet power to drive this cosmic renovation,” Wold said in the statement.

Galaxy 41028 (the green oval at center), has an estimated stellar mass of just 2 million suns, which is comparable to the largest star clusters in our Milky Way. (Image credit: NASA/ESA/CSA/Bezanson et al. 2024 and Wold et al. 2025)

Today, similar primitive galaxies, such as so-called “green pea” galaxies, are rare but known to release roughly 25% of their ionizing UV radiation into surrounding space. If early galaxies functioned in the same way, Wold said, they would have generated enough light to reionize the hydrogen fog and make the universe transparent.

“When it comes to producing ultraviolet light, these small galaxies punch well above their weight,” he said in the statement.

Astronomers have found a filament of hot gas, ten times as massive as our galaxy, that they reckon could explain where at least some of the universe’s “missing” matter might be lurking.

‘I guess NASA doesn’t need or care about my work anymore’ READ MORE

A third of “normal” matter in the universe is “missing.” It’s needed to make scientists’ models of the cosmos operate as postulated, but has proven difficult to find. The material is the ordinary stuff known as baryonic matter (baryons include protons and neutrons and other subatomic particles that make up the visible universe – not to be confused with dark matter or dark energy.) Physicists put the mass ratio of dark matter to baryonic matter at 5 to 1, meaning only approximately 15 or 16 percent of matter in the universe is normal matter. And according to a recent Nature Astronomy paper, only a “small fraction of baryons are in stars and the interstellar medium within galaxies.”

But boffins have suggested that the elusive matter might be found in long strings – or filaments – of gas, bridging pockets of space.

“While we’ve spotted filaments before,” said the European Space Agency (ESA), “it’s tricky to make out their properties; they’re typically faint, making it difficult to isolate their light from that of any galaxies, black holes, and other objects lying nearby.”

However, with data from ESA’s XMM-Newton and the Japan Space Agency’s (JAXA) Suzaku X-ray space telescopes, astronomers have found an enormous filament of hot gas that bridges four galaxy clusters.

As well as containing approximately ten times the mass of the Milky Way and stretching for 23 million light-years (equivalent to traversing the Milky Way end to end around 230 times), the filament clocks at over ten million degrees.

Finding and accurately characterizing a single filament of hot gas stretching between clusters of galaxies is quite the achievement. “For the first time, our results closely match what we see in our leading model of the cosmos – something that’s not happened before,” said lead researcher Konstantinos Migkas of Leiden Observatory in the Netherlands. “It seems that the simulations were right all along.”

ESA noted, “It also sheds light on the very nature of the ‘cosmic web’, the vast, invisible cobweb of filaments that underpins the structure of everything we see around us.”

While JAXA’s Suzaku X-ray telescope came to the end of its scientific mission in 2015, XMM-Newton remains operational more than a quarter of a century since its launch; a testament to its designers and the engineers running the mission. According to ESA, “Suzaku mapped the filament’s faint X-ray light over a wide region of space, while XMM-Newton pinpointed very precisely contaminating sources of X rays – namely, supermassive black holes – lying within the filament.”

“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,” said Norbert Schartel, ESA XMM-Newton Project Scientist.

Schartel told The Register that XMM-Newton had spotted evidence of the material before but said, “There we saw the missing matter in absorption, now we see it in emission.”

Schartel added that observing further filaments would be challenging, but noted that there were concepts for future X-ray satellites “with the specific aim to observe such filaments.” ®

A group of scientists may have solved one of the biggest mysteries in the Universe, following the discovery of a vast, glowing thread of gas bridging four galaxy clusters in deep space.

Stretching 23 million lightyears and burning at over 10 million°C, the filament could provide the answer to an enduring question in astronomy: where is Universe’s missing matter?

Despite all the stars, planets and galaxies that we can directly observe, astronomers say over a third of the Universe’s normal matter — the ‘stuff’ that makes up everything we see and know — is missing.

This isn’t a reference to dark matter, or dark energy, which are inferred but can’t be seen.

The missing matter is part of the stuff we can see, and the problem has been puzzling scientists for decades.

Simulation of the ‘cosmic web’, the vast network of threads and filaments that extends throughout the Universe. Credit: ESA

Discovering the missing matter

Scientists made the discovery using two powerful space telescopes: the European Space Agency’s XMM-Newton and Japan’s Suzaku.

Models of the Universe predict this ordinary matter should be out there, strung across space in long strings of gas – filaments – bridging gaps between the densest pockets of matter in the Universe.

These cosmic bridges have been seen before, but astronomers say they’re faint, making it difficult to see them among the glare of bright objects like galaxies, black holes and other nearby objects.

But this new study is one of the first to do this, finding and characterising a single filament of hot gas stretching between four clusters of galaxies 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,” says lead researcher Konstantinos Migkas of Leiden Observatory in the Netherlands.

“It seems that the simulations were right all along.”

Image showing a vast filament connecting four galaxy clusters: two on one end, two on the other. The filament contains a chunk of ‘missing matter’ that astronomers have been searching for. Credit: ESA/XMM-Newton and ISAS/JAXA. Acknowledgements: Migkas et al. (2025). Credit: ESA/XMM-Newton and ISAS/JAXA. Acknowledgements: Migkas et al. (2025)

A thread through the cosmos

The filament is 10 times as massive as our Milky Way galaxy and connects four galaxy clusters – two on one end, two on the other – all part of the Shapley Supercluster, a group of over 8,000 galaxies and one of the biggest structures in the nearby Universe.

XMM-Newton allowed scientists to pinpoint and remove bright sources of X-ray — like those from distant supermassive black holes — that would otherwise have muddied the view.

Credit: ESA/XMM-Newton and ISAS/JAXA. Acknowledgements: Migkas et al. (2025)

“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,” says co-author Florian Pacaud of the University of Bonn, Germany.

“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.”

Suzaku provided a broad map of the X-ray glow emitted by the hot gas, showing just how vast and dense the filament really is.

Chart showing the breakdown of ‘ordinary’ matter in the Universe. A newly discovered filament of hot intergalactic gas (here in mottled black-yellow) is a type of ordinary matter that has proven hard to find. Credit: ESA

Uncovering the foundations of the Universe

This filament is a key piece of evidence supporting how scientists think the Universe is built.

It shows how some of the densest, most massive structures in the Universe – galaxy clusters – are connected over huge distances across the cosmos.

And the fact the discovery lines up with theoretical models of cosmic evolution enables scientists to further assume their models are correct.

It’s also greater insight into the nature of the ‘cosmic web’ the vast, invisible cobweb of filaments that underpins the structure of the observable Universe.

An artist’s impression of XMM-Newton. Credit: ESA-C. Carrie

“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, 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.”

Euclid to follow

Artist’s impression of the Euclid spacecraft observing the dark Universe. Credit: ESA/Euclid/Euclid Consortium/NASA. Background galaxies: NASA, ESA, and S. Beckwith (STScI) and the HUDF Team, CC BY-SA 3.0 IGO

The European Space Agency’s Euclid mission, which has already begun releasing its first images and science data, is tasked with investigating dark matter and dark energy, which make up 95% of the Universe, but have never directly been observed.

Euclid is also tasked with uncovering more information about the cosmic web, so this latest discovery is a step in the right direction.

Euclid’s Deep Field South, in which the space telescope spotted over 11 million galaxies in one observation. In the coming years, Euclid will make more observations of this field. The image also reveals a glimpse of the ‘cosmic web’, the large-scale structure of the Universe. Credit: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre, E. Bertin, G. Anselmi

So where is the Universe’s missing matter?

It may, it seems, be glowing faintly in the filaments of the cosmic web, stretched across space like strands of an invisible spider’s silk.

And this latest discovery is one more step along the road to understand exactly the foundations of the cosmos.