"We thought it was the instrument": Scientists shocked by rare "Einstein cross" with a surprise at the center

This distinctive “Einstein cross” shows light from a distant galaxy, magnified and repeated five times instead of the usual four. (Image courtesy of Nicholas Lyra Turpo (ALMA) and adapted from Cox et al., 2025.)

A bizarre image of a distant galaxy has revealed what may be a massive clump of dark matter hiding in plain sight.

When astronomers first looked at a new image of the galaxy HerS-3 taken by the Northern Extended Millimeter Array radio telescopes in France, they thought there was a mistake in the data.

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The image showed what appeared to be an “Einstein cross.” This rare phenomenon occurs when light from a distant galaxy or quasar (a bright galaxy with a feeding black hole at its center) is refracted around the gravitational field of a massive object in front of it, so that from an observer's perspective, it appears to be split into four points. But what made this configuration unusual was the fifth point of light glowing at its center. At first, “we thought it was an instrumental issue,” Cox said.

Because the photons of an Einstein cross bend around the central mass, scientists don't typically expect to see a butt in the middle.

“You can't get a fifth image in the center unless there's something unusual going on with the mass refracting the light,” said Charles Keeton, an astronomer at Rutgers University and co-author of a new study describing the findings.

In a study published September 16 in The Astrophysical Journal, researchers used computer modeling to attempt to pinpoint the nature of the strange cross. Analysis revealed that all the glowing dots originated from HerS-3, ruling out interference from a closer, brighter object. They also ruled out a simple instrument malfunction by comparing the image with data obtained by the Large Millimeter/submillimeter Array (ALMA) in Chile.

Finally, they ran a computer simulation in which the dark matter mass was located in front of HerS-3—and this time the results matched their observations.