A massive galaxy cluster dating back to an era when such enormous structures were only beginning to take shape is challenging scientists’ understanding of how the universe evolved. New observations from NASA’s James Webb Space Telescope (JWST) reveal that the cluster contains an unusually dense concentration of matter, suggesting that some cosmic structures may have developed much faster than current models predict.
The galaxy cluster, known as XLSSC 122, has become the focus of three recent studies led by researchers at IPAC, Caltech’s science and data center for astrophysics and planetary science. Their findings establish XLSSC 122 as the most distant galaxy cluster known to exhibit strong gravitational lensing, a phenomenon that allows astronomers to measure the mass of otherwise invisible matter. The groundbreaking research was presented at a press conference on June 17, 2026, during the 248th meeting of the American Astronomical Society.
Located approximately 10.4 billion light-years from Earth, XLSSC 122 stood out to astronomers because of its surprising maturity. Although it existed during a period when galaxy clusters were still actively assembling, it already displayed the size and organized structure typically associated with mature clusters in the modern universe.
The powerful capabilities of the Webb telescope have now uncovered another remarkable feature that sets this cluster apart. XLSSC 122 happens to lie directly in front of one or more galaxies that are situated even farther away in the deep cosmos. The cluster’s immense gravity bends and distorts the light traveling from those background galaxies, creating a rare and visually striking effect known as strong gravitational lensing. This fortunate cosmic alignment has given scientists an exceptionally precise way to measure how mass is distributed throughout the cluster down to its core.
"When we got those first images back from JWST, we said, ‘wow, look at this, there’s strong lensing coming from this cluster!’" said Kyle Finner, a staff scientist at IPAC and lead author of the first paper examining the cluster. "XLSSC 122 has now set the record for the most distant galaxy cluster displaying strong lensing, which is a valuable tool for astronomers."
An Ancient Galaxy Cluster Challenges Cosmic Predictions
The newly detected gravitational lensing has allowed researchers to examine the distribution of mass in an ancient galaxy cluster in greater detail than ever before possible. XLSSC 122 dates to a crucial period known to astronomers as "cosmic noon," which occurred roughly 10 billion years ago when the universe was experiencing its most intense and prolific era of star formation.
During this dynamic epoch, galaxies were producing new stars at rates up to 100 times higher than those observed in the present-day universe. At the same time, massive galaxy clusters were becoming increasingly common as smaller systems gradually combined and merged into larger, more complex structures under the pull of gravity.
Yet XLSSC 122 appears to have developed in a significantly unexpected way. The new measurements show that a remarkably large amount of its total mass is packed tightly into its central region. According to standard cosmological models, the growth and centralization of massive cosmic structures should have been much more gradual, making such a dense and concentrated cluster particularly surprising at this early stage of cosmic history.
"XLSSC 122 is one of the first clusters we know of that formed in the universe, and it has a mass concentration that doesn’t agree with our cosmological model predictions," noted Finner.
To investigate this deep cosmic mystery further, Finner and his collaborators conducted two additional JWST studies, which were first authored by Zachary Scofield and Hyungjin Joo at Yonsei University. Together, the three comprehensive papers provide a much more complete picture of this unusual cluster and offer entirely new insights into the mechanisms of galaxy cluster formation during cosmic noon.
James Webb Reveals Hidden Dark Matter
Astronomers originally identified XLSSC 122 back in 2014 through an extensive X-ray survey carried out by the European Space Agency’s XMM-Newton spacecraft. Later observations conducted with the Hubble Space Telescope helped establish its remarkable distance of approximately 10.4 billion light-years and confirmed that the cluster appeared unusually well-developed for its tender age.
However, Hubble did not reveal clear evidence of strong gravitational lensing. That limitation changed dramatically when JWST captured distinctive curved arcs of light surrounding the cluster’s center, providing the first definitive and undeniable signs of this gravitational phenomenon in XLSSC 122.
"Before JWST, we couldn’t do this level of science in the early, distant universe," Finner said.
The dramatic gravitational distortions are caused primarily by something astronomers cannot directly observe or image: dark matter.
Although XLSSC 122 contains visible stars, hot plasma, and glowing gas, these familiar forms of ordinary matter account for only a small fraction of the total gravitational lensing effect. The vast majority of the distortion comes from dark matter, an invisible and elusive substance that has not been directly detected through emitted or reflected light, but whose profound gravitational influence is strongly supported by decades of extensive astronomical and cosmological evidence.
Scientists estimate that dark matter is approximately five times more abundant than ordinary matter, which makes up stars, planets, and everything else in the observable universe. Its pervasive gravity helps hold individual galaxies together and shapes the universe’s enormous cosmic web of galaxy groups, clusters, and expansive filaments.
Because dark matter plays such a foundational role in cosmic structure, determining how it was distributed billions of years ago offers researchers a powerful way to test and refine the theories describing the universe’s evolution since the Big Bang, which occurred approximately 13.8 billion years ago.
"Strong lensing is a way to measure the dark matter without actually seeing the dark matter," Finner explained. "It gives us a sensitive probe of our cosmological models."
A Wider View Reveals a Galaxy Cluster Merger
In their second dedicated paper, the research team investigated XLSSC 122 using another subtle form of gravitational lensing known as weak lensing.
While strong gravitational lensing produces obvious, dramatic distortions—including the striking luminous arcs captured by JWST—weak lensing causes much smaller, barely noticeable changes in the apparent shapes of distant background galaxies. These subtle effects are extremely difficult to identify on an individual basis, requiring astronomers to rely on sophisticated statistical methods across large fields of view to detect them.
The two complementary techniques provide vital information that bridges different scales. Strong gravitational lensing is especially useful for measuring the mass concentrated near a cluster’s dense center, whereas weak lensing can successfully reveal how matter is distributed across its vast outer regions and periphery.
"Weak gravitational lensing can constrain mass much further out, so you can get a better picture of the surrounding cluster area," Finner said.
By combining these precise mass measurements with comprehensive X-ray and radio observations gathered from other premier telescopes, the scientists determined that XLSSC 122 is actively undergoing a major merger. Its component galaxies are continuing to come together and interact as the larger cluster takes its ultimate shape.
This broader analysis also reinforced the earlier finding that an unusually large amount of mass is densely concentrated near the very center of the cluster, deepening the questions surrounding its rapid evolution.
Ancient Starlight Offers New Clues to Dark Matter
The team’s third and most recent paper revealed yet another important feature of XLSSC 122: faint, diffuse light produced by stars that are no longer gravitationally bound to any individual galaxy.
Known scientifically as intracluster light, this faint glow emanates from stars that have been stripped and scattered into the vast spaces between galaxies. Webb’s high-sensitivity observations represent the earliest known detection of intracluster light to date, providing yet another independent way to examine the assembly and development of galaxy clusters in the distant universe.
The widespread distribution of this ambient light strongly supports the conclusion that XLSSC 122 is actively undergoing a complex merger. As galaxies interact, pass close to one another, and collide within the cluster, external gravitational forces pull some stars away from their parent galaxies, dispersing them throughout the intergalactic space of the cluster. In XLSSC 122, many of these displaced stars appear not to have settled into its dense central region yet.
The researchers also uncovered an intriguing and potentially groundbreaking connection between these wandering stars and invisible dark matter.
Near the core of the cluster, the spatial distribution and shape of the intracluster light closely match the distribution of dark matter inferred independently from strong gravitational lensing. This striking correlation suggests that the faint glow of stars scattered between galaxies could potentially serve as a visible, accessible indicator of where invisible dark matter is concentrated.
If future astronomical observations confirm the exact same relationship in other distant clusters, particularly those from the early universe, intracluster light could quickly become a valuable new observational tool for mapping dark matter across cosmic history.
"In this cluster, the intracluster light essentially traces the dark matter," Finner said. "That light tells us that the cluster is in a merging state."
Could More Ancient Galaxy Clusters Challenge Cosmology?
The primary goal for Finner and his dedicated colleagues moving forward is to identify and investigate dozens of additional galaxy clusters from similarly early periods in the universe’s history.
Finding these distant, elusive objects requires a different observational strategy from studying them in high detail. Although the James Webb Space Telescope is exceptionally powerful, its relatively narrow field of view makes it far better suited to examining selected targets rather than conducting blind searches across enormous areas of the sky.
Instead, astronomers must first identify promising candidate clusters through wide-area X-ray and radio surveys, similar to the legacy observations that originally revealed XLSSC 122 over a decade ago.
Another highly promising technique relies on the Sunyaev-Zel’dovich effect, a phenomenon that occurs when highly energetic electrons inside massive galaxy clusters scatter radiation originating from the cosmic microwave background, which serves as the faint thermal afterglow of the Big Bang.
This interaction can create distinctive shadow regions that appear as recognizable "holes" or depressions in high-resolution observations of the cosmic microwave background. By searching systematically for these specialized signatures, astronomers can locate distant galaxy clusters that might otherwise remain entirely hidden.
Discovering more objects comparable to XLSSC 122 could have major, far-reaching implications for modern cosmology. If other ancient clusters also contain unexpectedly concentrated dark matter and display similarly advanced development at early epochs, scientists may need to revise some of their foundational assumptions regarding how rapidly large-scale cosmic structures could form in the young universe.
For now, XLSSC 122 provides a rigorous and vital test of existing theoretical models, but a much larger sample size will be strictly necessary to determine whether its unusual properties are extremely rare anomalies or part of a broader, unrecognized pattern in the early universe.
"It’s still early in the JWST era," Finner concluded, "and if we can start to get data on tens or hundreds of these types of objects at this stage in the universe, then we can really start putting our cosmological models to the test."