JWST reveals ancient galaxy cluster with dense dark matter

Researchers led by Kyle Finner presented findings at the American Astronomical Society’s 248th meeting on June 17, 2026, showing that JWST images revealed strong gravitational lensing arcs—the bending of light by mass—around the galaxy cluster XLSSC 122. XLSSC 122 resides roughly 10.4 billion light-years from our planet.

XLSSC 122 was first detected in 2014 by an X-ray survey conducted with ESA’s XMM-Newton spacecraft. Later Hubble Space Telescope observations confirmed its distance and showed the cluster appeared unusually mature for its epoch.

JWST images provided the first definitive evidence of strong gravitational lensing arcs—the bending of light by mass—around XLSSC 122. XLSSC 122 now holds the title of the most distant galaxy cluster displaying strong gravitational lensing.

Analysis of strong lensing shows that a significant mass concentration lies in the core of XLSSC 122. The central mass concentration exceeds the expectations of standard cosmological models for a cluster at cosmic noon, an era about 10 billion years ago when star formation was at its peak.

Dark matter, an invisible form of matter that exerts gravity, accounts for the majority of the gravitational lensing signal in XLSSC 122, while visible stars and gas contribute only a small fraction. JWST detected intracluster light—a faint glow of stars not bound to individual galaxies—in XLSSC 122, marking the earliest known observation of such light. The spatial distribution of intracluster light in XLSSC 122 closely matches the dark matter distribution inferred from strong lensing near the cluster’s center.

Weak lensing (the bending of light by mass on larger scales) and multiwavelength data (data across multiple wavelengths) show that XLSSC 122 is still undergoing a galaxy cluster merger. Kyle Finner, a staff scientist based at IPAC, which is Caltech’s facility for science and data in astrophysics and planetary science, directed the study of XLSSC 122. The team shared their results at a press conference held on June 17, 2026, during the 248th gathering of the American Astronomical Society.

XLSSC 122 belongs to the era called cosmic noon, a time when star formation reached levels up to 100 times greater than today. The researchers suggest that the unusually dense dark matter in XLSSC 122 implies early‑universe dark‑matter halos may be far denser than predicted, offering new constraints on galaxy formation and dark‑matter physics.