James Webb reveals a massive cosmic structure that challenges our understanding of the Universe
NASA’s James Webb Space Telescope has revealed an ancient galaxy cluster with an unexpectedly dense concentration of dark matter, challenging theories about how the universe developed. The discovery also uncovered a surprising connection between scattered starlight and invisible dark matter that could transform how astronomers study the early cosmos.
Key points
- JWST found strong gravitational lensing in XLSSC 122, the most distant galaxy cluster known to show it.
- The cluster's mass is unusually concentrated at its center, conflicting with cosmological model predictions.
- Webb detected the earliest known intracluster light, which traces dark matter in the merging cluster.
New James Webb Space Telescope observations of a galaxy cluster called XLSSC 122 are challenging scientists' understanding of how the universe evolved. The cluster, about 10.4 billion light-years from Earth, contains an unusually dense concentration of matter, suggesting some cosmic structures developed faster than current models predict.
The findings come from three recent studies led by researchers at IPAC, Caltech's science and data center for astrophysics and planetary science. The research was presented at a press conference on June 17, 2026, during the 248th meeting of the American Astronomical Society.
Record gravitational lensing
XLSSC 122 is now the most distant galaxy cluster known to exhibit strong gravitational lensing, in which the cluster's gravity bends light from galaxies behind it. This alignment lets astronomers measure the mass of otherwise invisible matter. The cluster dates to "cosmic noon," roughly 10 billion years ago, when the universe had its most intense star formation.
Astronomers first identified XLSSC 122 in 2014 through an X-ray survey by the European Space Agency's XMM-Newton spacecraft. Hubble later established its distance and showed it looked unusually developed for its age, but did not reveal clear strong lensing. JWST captured distinctive curved arcs of light around the cluster's center, the first definitive signs of the phenomenon.
XLSSC 122 has now set the record for the most distant galaxy cluster displaying strong lensing, which is a valuable tool for astronomers.
— Kyle Finner
Dark matter and merging stars
Most of the lensing distortion comes from dark matter, an invisible substance not directly detected through light. Scientists estimate dark matter is about five times more abundant than ordinary matter. The team's second paper used weak lensing to study mass in the cluster's outer regions, and combined measurements showed XLSSC 122 is still undergoing a merger.
A third paper revealed faint intracluster light from stars no longer bound to individual galaxies, the earliest known detection of such light. Near the cluster's center, its shape closely matches the dark matter distribution inferred from strong lensing. Finner said the next goal is to find dozens more early clusters to test cosmological models.
What researchers said
Kyle Finner, a staff scientist at IPAC and lead author of the first paper, said the team reacted with surprise when the first JWST images showed strong lensing from the cluster. He said XLSSC 122 has set the record for the most distant galaxy cluster displaying strong lensing, calling it a valuable tool for astronomers.
Finner said the cluster's mass concentration does not agree with cosmological model predictions. He also said strong lensing measures dark matter without actually seeing it, and that intracluster light in this cluster essentially traces the dark matter.
