Jens Asbjørn Bøgen
The James Webb Space Telescope keeps finding things in the early universe that are not supposed to be there yet.
Every few months, a new picture from Webb lands on an astrophysicist’s desk and sends them back to a whiteboard full of equations about how galaxies, stars, and clusters are meant to grow.
Lately, the awkward objects keep piling up on the same shelf: structures that look older, denser, and more organized than cosmology’s standard timeline allows.
Three peer-reviewed studies now put a name to the latest problem child. The galaxy cluster XLSSC 122, observed 10.4 billion light-years away, has a far more concentrated central mass than the standard model of structure formation predicts, according to a team led by IPAC staff scientist Kyle Finner.
The findings were presented on June 17, 2026, at the 248th meeting of the American Astronomical Society.
The cluster sits at redshift z = 1.98, in the era astronomers call cosmic noon, the stretch roughly 10 billion years ago when the universe was running at full tilt and stars were forming up to 100 times faster than they do today. Galaxy clusters are only just supposed to be assembling in that period, not acting like finished products.
A test that measures dark matter without seeing it
XLSSC 122 was first picked up in 2014 by the XMM-Newton X-ray observatory. What Webb added is strong gravitational lensing: giant arcs of light from more distant galaxies, warped by the cluster’s gravity into a near-perfect probe of its mass.
“When we got those first images back from JWST, we said, ‘Wow, look at this, there’s strong lensing coming from this cluster!'” Finner said in the Caltech/IPAC release. The cluster is now the most distant known to display the effect.
“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.” Dark matter outweighs ordinary matter in the cluster by roughly five to one, and lensing is one of the few ways to map it directly.
A second paper, led by Zachary Scofield of Yonsei University, used weak lensing to track mass across the cluster’s outer regions, and found evidence that XLSSC 122 is still swallowing other groups of galaxies.
As Finner put it, the cluster “is in a merging state.”
The earliest glow between galaxies
The third study, led by Hyungjin Joo, also at Yonsei University, flagged something astronomers had never managed to catch this far back: a faint smear of starlight drifting in the space between the cluster’s galaxies. This intracluster light comes from stars ripped out of their home galaxies during collisions.
Its glow matters because of what it points at. “In this cluster, the intracluster light essentially traces the dark matter,” Finner said. If that holds up in other systems, the diffuse light could become a cheap way to chart the invisible matter that holds structures together across cosmic time.
The combined picture, from three separate techniques pointing at the same object, is why the researchers are willing to say XLSSC 122 does not fit the current playbook. “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 said.
What breaks, and what gets tested next
A single awkward object is a curiosity, not a crisis. Cosmologists need a sample before they revise the model that has carried modern astronomy for decades. That is where the team wants to take the work.
“It’s still early in the JWST era, 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,” Finner said.
The three papers are published in The Astrophysical Journal Letters.
