A rare cosmic configuration: An Einstein Cross with five points of light, instead of the usual four, has been discovered by scientists. Credit: Nicolás Lira Turpaud (ALMA Observatory) & adapted from Cox et al. 2025.

Astronomers have found a rare Einstein cross that will help them learn more about the invisible matter that holds the universe together.

When Rutgers theoretical astrophysicist Charles Keeton first saw an unusual picture shared by his colleague, he was intrigued.

“Have you ever seen an Einstein Cross with an image in the middle?” his colleague Andrew Baker asked, referring to a rarely seen cosmic configuration. Keeton hadn’t, and the implications were enormous.

“I said, well, that’s not supposed to happen,” said Keeton, the Vice Provost for Experiential Learning at Rutgers University-New Brunswick. “You can’t get a fifth image in the centre unless something unusual is going on with the mass that’s bending the light.”

An ‘Einstein Cross’ is a rarely seen cosmic configuration, in which the light from a distant galaxy is bent by the gravity of galaxies in front of it, creating four images. But the extra image in this Einstein Cross pointed to ‘something unusual’, which turned out to be a massive, hidden halo of dark matter.

The existence of this invisible structure could only be inferred through careful computer modelling and analysis.

Rare chance

Dark matter makes up most of the matter in the Universe, but it can’t be seen directly.

Study co-author Baker, a Distinguished Professor in the Department of Physics and Astronomy in the School of Arts and Sciences, said: “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.

“This discovery gives us a rare chance to study that invisible structure in detail.”

The first step toward that discovery was taken in France.

Study lead author Pierre Cox, a French astronomer, Research Director at the French National Centre for Scientific Research, who first spotted the anomaly in data from the Northern Extended Millimeter Array (NOEMA) of radio telescopes in the French Alps, said: “It looked like a cross, and there was this image in the centre. I knew I had never seen that before.”

The team was studying a distant, dusty galaxy called HerS-3. Using NOEMA and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, they saw that the light from HerS-3 split into five rather than four images.

At first, they thought it might be a glitch in the data, but the fifth image wouldn’t go away.

Cox said: “We tried to get rid of it. We thought it was a problem with the instrument. But it was real.”

Dark matter halo

Computer modelling of the gravitational lens by Keeton and Rutgers graduate student Lana Eid showed that the four visible foreground galaxies causing the gravitational bending couldn’t explain the details of the five-image pattern.

Only with the addition of a large, invisible mass, in this case, a dark matter halo, could the model match the observations.

Co-author Keeton said: “We tried every reasonable configuration using just the visible galaxies, and none of them worked.

“The only way to make the math and the physics line up was to add a dark matter halo.

“That’s the power of modelling; it helps reveal what you can’t see.”

The unusual configuration doesn’t just look cool: the scientists said it’s scientifically valuable.

The lensing effect magnifies the background galaxy, allowing astronomers to study its structure in greater detail than usual.

It also offers a rare chance to learn about the dark matter that surrounds the foreground galaxies.

Cox said: “This system is like a natural laboratory. We can study both the distant galaxy and the invisible matter that’s bending its light.”

Co-author Eid, who is pursuing her doctoral degree, said her involvement in the research project has been exciting from beginning to end: “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.

“Collaborating across continents and time zones taught me the value of diverse expertise and research styles in fully understanding a new discovery.”

Future observations

The team has even predicted that more features, such as outflowing gas from the galaxy, could be visible in future observations.

If those predictions are confirmed, it would be a powerful validation of their models. If not, it would still teach them something new.

Keeton said: “This is a falsifiable prediction; if we look and don’t see it, we’ll have to go back to the drawing board. That’s how science works.”

The study is published in The Astrophysical Journal.