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 cosmological 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 clusters found in the modern universe.
The powerful infrared capabilities of the Webb Space Telescope have now uncovered another remarkable feature regarding its alignment in space. XLSSC 122 happens to lie directly in front of one or more galaxies that are situated even farther away in the deep cosmos. The massive cluster’s intense gravity bends and distorts the light traveling from those background galaxies, creating a rare and visually striking effect called strong gravitational lensing. This fortunate cosmic alignment has given scientists an exceptionally precise way to measure how mass is distributed throughout the interior of the cluster.
"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 seeking to understand the early universe."
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 back to a critical period known as "cosmic noon," roughly 10 billion years ago, when the universe was experiencing its most intense and prolific era of star formation.
During this dynamic time in cosmic history, galaxies were producing new stars at rates up to 100 times higher than those observed in the present-day universe. Galaxy clusters were also 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 an unexpected and accelerated 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 of massive structures should have been much more gradual, making such a concentrated and advanced 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 compelling 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 how galaxy clusters formed during the height of cosmic noon.
James Webb Reveals Hidden Dark Matter
Astronomers originally identified XLSSC 122 back in 2014 through a targeted X-ray survey carried out by the European Space Agency’s XMM-Newton spacecraft. Later follow-up observations with the Hubble Space Telescope helped establish its immense distance of approximately 10.4 billion light-years and confirmed that the cluster appeared unusually well-developed for its chronological age.
However, Hubble observations did not reveal clear, definitive evidence of strong gravitational lensing. That scientific barrier changed dramatically when JWST captured distinctive curved arcs of light surrounding the cluster’s dense center, providing the first undeniable signs of this optical phenomenon in XLSSC 122.
"Before JWST, we couldn’t do this level of science in the early, distant universe," Finner emphasized.
The dramatic gravitational distortions are caused primarily by something astronomers cannot directly observe or measure with standard telescopes: dark matter.
Although XLSSC 122 contains visible stars, glowing gas, and cosmic dust, 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 elusive, invisible substance that has not been directly detected through emitted light, but whose profound gravitational influence is heavily supported by decades of extensive astronomical evidence.
Scientists estimate that dark matter is approximately five times more abundant in the universe than ordinary matter, which makes up stars, planets, and everything we interact with daily. Its pervasive gravity helps hold galaxies together and shapes the universe’s enormous, web-like network of galaxy groups, clusters, and filaments.
Because dark matter plays such an essential role in cosmic structure and evolution, determining how it was distributed billions of years ago offers a powerful way to test the theoretical models describing the universe’s development since the Big Awakening of the Big Bang 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 visual distortions, including the striking, bright arcs captured by JWST, weak lensing causes much smaller, nearly imperceptible 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 to detect them across a wide field.
The two complementary techniques provide distinct pieces of information. Strong gravitational lensing is especially useful for measuring the mass heavily concentrated near a cluster’s center, while weak lensing can reveal how matter is distributed across its vast, diffuse outer regions.
"Weak gravitational lensing can constrain mass much further out, so you can get a better picture of the surrounding cluster area," said Finner.
By combining these precise measurements with X-ray and radio observations obtained from other space- and ground-based 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 final shape.
The broader analysis also reinforced the earlier finding that an unusually large amount of mass is densely concentrated near the core of the cluster, deepening the questions surrounding its rapid development.
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 bound to any individual galaxy.
Known as intracluster light, this faint glow comes from ancient stars that have been stripped and scattered into the vast spaces between galaxies. Webb’s high-resolution observations represent the earliest known detection of intracluster light, providing 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, graze, and collide within the cluster, some stars are gravitationally pulled away from their parent galaxies and spread throughout the intergalactic medium. In XLSSC 122, many of these displaced stars appear not to have settled into its dense central region.
The researchers also uncovered an intriguing and potentially vital connection between these wandering stars and invisible dark matter.
Near the cluster’s center, the spatial shape and distribution of the intracluster light closely matches the distribution of dark matter inferred independently from strong gravitational lensing. This striking correlation suggests that the faint glow of stars residing between galaxies could potentially serve as a visible, reliable indicator of where invisible dark matter is concentrated.
If future astronomical observations confirm the exact same relationship in other clusters, particularly those originating from the early universe, intracluster light could 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 immediate next goal for Finner and his dedicated colleagues is to identify and investigate dozens of additional galaxy clusters from similarly early periods in the universe’s history.
Finding these distant and faint objects requires a different strategic approach from studying them in detail. Although the James Webb Space Telescope is exceptionally powerful, its relatively narrow field of view makes it much better suited to examining carefully selected targets than searching vast, sweeping areas of the sky.
Instead, astronomers must first identify promising candidates through wide-area X-ray and radio surveys, similar to the legacy observations that originally revealed XLSSC 122 more than a decade ago.
One promising technique relies on the Sunyaev-Zel’dovich effect, a phenomenon that occurs when energetic, high-energy particles inside massive galaxy clusters scatter radiation from the cosmic microwave background, which is the faint, persistent afterglow of the Big Bang.
This physical interaction can create distinctive regions that appear as cooler "holes" in observations of the cosmic microwave background. By systematically searching for these specialized signatures, astronomers can locate distant galaxy clusters that might otherwise remain hidden or difficult to detect.
Finding more ancient objects like XLSSC 122 could have major implications for modern cosmology. If other ancient clusters also contain unexpectedly concentrated dark matter and display unusually advanced development for their age, scientists may need to revise some of their most fundamental assumptions about how quickly large cosmic structures formed in the early universe.
For now, XLSSC 122 provides an important and rigorous test of existing theoretical models, but a larger sample size will ultimately be necessary to determine whether its unusual properties are extremely rare anomalies or part of a much broader, overlooked 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."
