When Rutgers theoretical astrophysicist Charles Keeton first received an unusual picture from his colleague Andrew Baker, his curiosity was immediately piqued.
“Have you ever seen an Einstein Cross with an image in the middle?” Baker asked.
Keeton hadn’t. And when he looked at the picture, his mind leapt. Something about it was deeply unsettling.
“I said, well, that’s not supposed to happen,” recalled Keeton, now Vice Provost for Experiential Learning at Rutgers University–New Brunswick. “You can’t get a fifth image in the center unless something unusual is going on with the mass that’s bending the light.”
What he was looking at wasn’t just a striking picture from the cosmos. It was a puzzle that could only be solved with an invisible piece of the universe: dark matter.
What Is an Einstein Cross?
An Einstein Cross is one of the most dazzling cosmic illusions created by the phenomenon known as gravitational lensing. Imagine a distant galaxy, billions of light-years away, sending out light across space. If another galaxy—or several—lies between it and Earth, the gravity of those foreground galaxies bends and magnifies the background light, almost like a giant cosmic lens.
Sometimes, the geometry is just right. Instead of seeing one image of the distant galaxy, astronomers see four duplicates arranged in a cross shape around the foreground galaxy. This is the Einstein Cross, a phenomenon first predicted by Albert Einstein’s general theory of relativity.
But the version spotted by Keeton’s team was different. It didn’t show just four images. It showed five. And that fifth one, glowing faintly in the center, was something extraordinary.
A Discovery in the French Alps
The first hints of this cosmic anomaly appeared not in New Jersey but in the French Alps. Pierre Cox, a French astronomer and Research Director at the French National Center for Scientific Research, was poring over data from the Northern Extended Millimeter Array (NOEMA)—a powerful set of radio telescopes nestled high in the mountains.
“We were like, ‘What the heck?’” Cox recalled.
The data showed the familiar cross-like pattern of four images, but in the middle, a ghostly extra image gleamed. Cox was baffled. “It looked like a cross, and there was this image in the center. I knew I had never seen that before.”
The team was studying a distant, dusty galaxy called HerS-3, its light stretched and distorted by the gravitational influence of closer galaxies. At first, Cox and his colleagues assumed the central image must be an error. Perhaps a glitch in the telescope, some artifact of the data.
“We tried to get rid of it,” Cox said. “We thought it was a problem with the instrument. But it was real.”
The Hidden Hand of Dark Matter
The problem with the data was not the telescope—it was the assumptions about what was doing the lensing. Keeton, along with Rutgers graduate student Lana Eid, built computer models of the gravitational lens. Their task was simple in theory: account for the shape of the images using only the visible galaxies.
But no matter how many ways they rearranged the known masses, the models failed. The four foreground galaxies weren’t enough to explain the fifth image.
“We tried every reasonable configuration using just the visible galaxies, and none of them worked,” Keeton explained. “The only way to make the math and the physics line up was to add a dark matter halo.”
Dark matter, the invisible scaffolding of the universe, doesn’t emit or absorb light. It can’t be seen directly. But its presence reveals itself through gravity—by the way it bends light and influences the motion of galaxies.
In this case, the computer models required an enormous, unseen halo of dark matter to create the fifth image. Without that invisible mass, the cosmic puzzle couldn’t be solved.
Why This Einstein Cross Matters
On the surface, the Einstein Cross with its mysterious fifth image looks like a celestial curiosity—a rare and beautiful optical illusion. But its scientific value is immense.
The lensing effect not only magnifies the distant galaxy, allowing astronomers to study its internal structure in detail; it also provides a rare opportunity to probe dark matter itself.
“This system is like a natural laboratory,” said Cox. “We can study both the distant galaxy and the invisible matter that’s bending its light.”
By observing how the galaxy’s light is distorted, astronomers can map the dark matter distribution around the foreground galaxies. Each new system like this adds to humanity’s growing picture of how dark matter shapes the cosmos.
The Thrill of Discovery
For Lana Eid, a graduate student at Rutgers and co-author of the study, the discovery was as personal as it was scientific.
“I was thrilled to join this project as a graduate student, especially since it involved a fascinating lensing system that grew more intriguing as our models evolved,” she said.
The international collaboration stretched across continents and time zones. Researchers from the U.S., France, and Chile worked together, combining expertise and data from multiple observatories, including NOEMA in France, ALMA in Chile, and the Hubble Space Telescope.
Eid described the experience as transformative: “Collaborating across continents and time zones taught me the value of diverse expertise and research styles in fully understanding a new discovery.”
A Prediction and a Test
Perhaps most excitingly, the team’s models don’t just explain the five-image configuration—they make predictions. According to Keeton and Eid, the galaxy HerS-3 should show signs of outflowing gas in future observations.
“This is a falsifiable prediction,” Keeton emphasized. “If we look and don’t see it, we’ll have to go back to the drawing board. That’s how science works.”
Such predictions are the essence of the scientific method: hypotheses must face the harsh light of observation. Whether the prediction is confirmed or not, the outcome will deepen astronomers’ understanding of the cosmos.
The Invisible Majority
This discovery also underscores a profound truth about our universe: most of it is invisible. Dark matter accounts for about 85% of all matter, yet we cannot see it. We know it only through its gravitational influence—how it bends light, shapes galaxies, and sculpts the cosmic web.
“Dark matter makes up most of the matter in the universe, but it can’t be seen directly,” said Baker. “We only know it’s there because of how it affects the things we can see, like the way it bends light from distant galaxies.”
To stumble upon an Einstein Cross with a central image is to catch a rare glimpse of that hidden structure. It is like finding the faint outline of a giant in the fog—not visible to the eye, but undeniable in its impact.
Science Without Borders
The discovery also highlights the global nature of modern astronomy. Observatories in the French Alps and the Chilean desert worked hand in hand with data analysis performed in New Jersey. U.S. federal support played a crucial role, funding the National Science Foundation’s Very Large Array (VLA) and NASA’s Hubble Space Telescope.
“These facilities are essential,” Baker said. “We hope they will continue to enable such discoveries well into the future.”
A Window Into the Unknown
The Einstein Cross with five images is more than an astronomical curiosity. It is a reminder of the universe’s capacity to surprise us, of the hidden forces shaping the cosmos, and of humanity’s relentless pursuit of knowledge.
What began with an astronomer in France saying, “What the heck?” became an international scientific breakthrough, revealing one more clue about the invisible matter that dominates the universe.
The fifth image shouldn’t have been there. And yet, it was. In that impossibility lay the key to something vast and unseen: a dark halo of matter, hidden but real, bending light across billions of years to reach our telescopes and imaginations.
And in the faint glow of that fifth image, humanity glimpses once more how much more there is to discover.
More information: P. Cox et al, HerS-3: An Exceptional Einstein Cross Reveals a Massive Dark Matter Halo, The Astrophysical Journal (2025). DOI: 10.3847/1538-4357/adf204
