Astronomers first picked out XLSSC 122 in 2014, through an X-ray survey by the European Space Agency’s XMM-Newton spacecraft. Hubble Space Telescope observations later placed the galaxy cluster approximately 10.4 billion light-years from Earth.
Now NASA’s James Webb Space Telescope has found curved arcs of light around the cluster’s centre, the first conclusive evidence of strong gravitational lensing there. The arcs show light bending around XLSSC 122’s unseen dark matter. The result makes XLSSC 122 the most distant known galaxy cluster with strong gravitational lensing.
The observations have revealed an ancient cluster with an unexpectedly dense concentration of dark matter. They also show a remarkably large amount of mass concentrated in its central region.
That density is difficult to square with standard cosmological models, which indicate that massive structures should have assembled more gradually at this early stage of cosmic history. If the cluster’s properties cannot be explained within current theory, scientists may have to reconsider how ordinary matter and dark matter evolved together over cosmic time.
XLSSC 122 dates from cosmic noon, roughly 10 billion years ago, when galaxies were making stars at rates up to 100 times higher than in the present-day universe. The finding therefore offers a view of a cluster that formed during one of the universe’s most active periods of star production.
Dark matter is a proposed form of matter that gives off no light and was introduced to account for gaps between the gravitational effects astronomers observe and the visible mass in the universe. It is approximately five times more abundant than ordinary matter. The Standard Model has no viable particle candidate to account for the gravitational effects assigned to dark matter. Suggested additions to that model include weakly interacting massive particles, Massive Compact Halo Objects and axions.
Kyle Finner, a staff scientist at IPAC, is lead author of the first paper examining XLSSC 122. Zachary Scofield and Hyungjin Joo, both at Yonsei University, are first authors of two further JWST studies of the cluster. Three recent studies of XLSSC 122 were led by researchers at IPAC, Caltech’s science and data centre 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.
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