A massive galaxy cluster dating back to a formative era of the universe is challenging scientists’ fundamental understanding of how cosmic structures evolved. New observations from NASA’s James Webb Space Telescope (JWST) reveal that the cluster, known as XLSSC 122, contains an unusually dense concentration of matter. This unexpected density suggests that some cosmic structures may have developed much faster during the early history of the universe than current theoretical models predict.
The galaxy cluster has become the focal point of three recent studies led by researchers at IPAC, Caltech’s science and data center for astrophysics and planetary science. Their findings firmly establish XLSSC 122 as the most distant galaxy cluster known to exhibit strong gravitational lensing, a powerful cosmic phenomenon that allows astronomers to precisely measure the mass of otherwise invisible matter. The groundbreaking research was formally 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 immediately 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 physical size and highly organized structure typically associated with mature clusters in the modern universe.
The James Webb Space Telescope has now uncovered another remarkable feature that elevates the cluster’s scientific value. XLSSC 122 happens to lie directly in front of one or more background galaxies that are situated even farther away in the distant cosmos. The cluster’s immense gravitational field 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 provided scientists with an exceptionally precise method to map and measure how mass is distributed throughout 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."
An Ancient Galaxy Cluster Challenges Cosmic Predictions
The newly detected gravitational lensing has allowed researchers to examine the distribution of mass within an ancient galaxy cluster in greater detail than ever previously possible. XLSSC 122 dates back to a pivotal period known 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 across the universe were producing new stars at rates up to 100 times higher than those observed in the present-day universe. At the same time, galaxy clusters were becoming increasingly common as smaller stellar systems and gas clouds gradually combined and merged into larger, more massive structures.
Yet XLSSC 122 appears to have developed in a remarkably unexpected way. The new measurements demonstrate that a surprisingly large amount of its total mass is tightly packed into its central region. According to standard cosmological models, the growth and consolidation of massive structures should have proceeded much more gradually, 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," Finner explained.
To investigate this deep astrophysical mystery further, Finner and his close collaborators conducted two additional JWST studies, which were first authored by Zachary Scofield and Hyungjin Joo at Yonsei University. Together, these three complementary papers provide a much more complete, multifaceted picture of this unusual cluster and offer profound 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 a targeted X-ray survey carried out by the European Space Agency’s XMM-Newton spacecraft. Later follow-up observations conducted 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 tender age.
However, Hubble observations did not reveal clear, definitive evidence of strong gravitational lensing. That limitation changed dramatically when the James Webb Space Telescope captured distinctive, curved arcs of light surrounding the cluster’s core, 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," noted Finner.
The dramatic gravitational distortions are caused primarily by something astronomers cannot directly observe or detect with traditional instruments: dark matter.
Although XLSSC 122 contains visible stars, dense dust lanes, and glowing hot 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 originates from dark matter, an elusive, invisible substance that has not been directly detected through emitted or absorbed light, but whose pervasive gravitational influence is supported by extensive astronomical evidence across the cosmos.
Scientists estimate that dark matter is approximately five times more abundant than ordinary matter, which makes up stars, planets, and everything in our everyday world. Its powerful gravity helps hold spinning galaxies together and shapes the universe’s enormous, web-like network of galaxy groups, clusters, and expansive filaments.
Because dark matter plays such a foundational role in cosmic structure and evolution, determining precisely how it was distributed billions of years ago offers a uniquely powerful way to test the core theories describing the universe’s development since 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 said. "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 vividly by JWST—weak lensing causes much smaller, imperceptible changes in the apparent shapes of distant background galaxies. These subtle effects are notoriously difficult to identify individually, requiring astronomers to rely heavily on advanced statistical methods across large fields of view to detect them.
The two distinct techniques provide vital complementary information. Strong gravitational lensing is especially useful for measuring the massive concentration of material near a cluster’s dense center, whereas weak lensing can reveal how matter is broadly distributed across its expansive outer regions.
"Weak gravitational lensing can constrain mass much further out, so you can get a better picture of the surrounding cluster area," Finner explained.
By combining these sophisticated gravitational measurements with independent X-ray and radio observations gathered by other space- and ground-based telescopes, the scientists determined that XLSSC 122 is actively undergoing a major merger. Its constituent galaxies are continuing to come together and interact as the larger, unified cluster takes physical shape.
This broader, multi-wavelength analysis also reinforced the earlier finding that an unusually large amount of mass remains heavily concentrated near the cluster’s central core.
Ancient Starlight Offers New Clues to Dark Matter
The team’s third and most recent paper revealed yet another important feature of XLSSC 122: extremely faint light produced by wandering stars that are no longer gravitationally bound to any individual galaxy.
Known as intracluster light, this diffuse, ghostly glow originates from stars that have been stripped and scattered into the vast, dark spaces between galaxies. Webb’s unprecedented observations represent the earliest known detection of intracluster light, providing yet another powerful way to examine the complex development of galaxy clusters in the distant universe.
The widespread, diffuse distribution of this light strongly supports the conclusion that XLSSC 122 is actively undergoing a merging process. As massive galaxies interact, pass close to one another, and collide, gravitational forces pull some stars away from their parent galaxies and disperse them throughout the broader cluster environment. In XLSSC 122, however, many of these displaced stars appear not to have settled into its dense central region.
The researchers also uncovered an intriguing, unexpected connection between these wandering stars and invisible dark matter.
Near the cluster’s dense center, the spatial shape and distribution of the intracluster light closely mirror the distribution of dark matter inferred independently from strong gravitational lensing. This striking correlation suggests that the faint, ghostly glow of stars drifting between galaxies could potentially serve as a visible, reliable indicator of where invisible dark matter is heavily concentrated.
If future astronomical observations confirm the exact same relationship in other clusters, particularly those originating from the early universe, intracluster light could quickly become a valuable new observational tool for mapping dark matter across deep cosmic history.
"In this cluster, the intracluster light essentially traces the dark matter," said Finner. "That light tells us that the cluster is in a merging state."
Could More Ancient Galaxy Clusters Challenge Cosmology?
The primary scientific goal moving forward for Finner and his colleagues is to identify, confirm, and investigate dozens of additional galaxy clusters from similarly early periods in the universe’s history.
Finding these distant, elusive objects presents a distinct observational challenge. Although the James Webb Space Telescope is exceptionally powerful, its relatively narrow field of view makes it far better suited for examining carefully selected targets in detail rather than searching vast, sweeping areas of the sky.
Instead, astronomers must first identify promising candidate clusters through wide-area X-ray and radio surveys, similar to the historical observations that originally revealed XLSSC 122 over a decade ago.
Another highly promising technique relies on the Sunyaev-Zel’dovich effect, which occurs when ultra-energetic electrons inside massive galaxy clusters scatter radiation from the cosmic microwave background—the faint, lingering thermal afterglow of the Big Bang.
This physical interaction can create distinctive regions that appear as recognizable "holes" or shadows in observations of the cosmic microwave background. By systematically searching for these unique signatures, astronomers can locate distant galaxy clusters that might otherwise remain entirely hidden.
Finding more objects resembling XLSSC 122 could carry major implications for modern cosmology. If other ancient clusters similarly contain unexpectedly concentrated dark matter and display unusually advanced development for their age, scientists may ultimately need to revise some of their most basic assumptions regarding how quickly large cosmic structures formed after the dawn of the universe.
For now, XLSSC 122 provides an invaluable, stringent test of existing astrophysical models, but a much larger sample size will be strictly necessary to determine whether its peculiar properties represent 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."
