An Exceptional Einstein Cross Reveals Hidden Dark Matter

Republished from ALMA, the Atacama Large Millimeter/submillimeter Array, where it first appeared as a press release on 2025-09-16. ALMA press-release texts and images are released under the Creative Commons Attribution 4.0 International licence; the text is reproduced here unchanged apart from formatting. Credit: ALMA. Read the original for the latest version and any updates.

An international team of astronomers, including researchers from ALMA, has discovered a spectacular Einstein Cross in the distant universe that reveals the hidden presence of dark matter. Observations used data from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the Northern Extended Millimeter Array (NOEMA) in France, the Karl G. Jansky Very Large Array (VLA) in the USA, and the NASA/ESA Hubble Space Telescope. The findings are now published in the Astrophysical Journal.

The galaxy, known as HerS-3, lies 11.6 billion light-years away and appears multiplied into five images by a massive group of galaxies located 7.8 billion light-years from Earth. This striking lensing effect, called an Einstein Cross, is scarce, and in this case, even more extraordinary because of the presence of a bright fifth image at the center of the cross.

The light from HerS-3 is bent by four massive foreground galaxies that sit at the core of a larger group containing at least ten more galaxies. However, detailed lensing models showed that the visible galaxies alone could not account for the exact arrangement of the five images.

“The only way to reproduce the remarkable configuration we observed was to add an invisible, massive component: a dark matter halo at the center of the galaxy group,” explains Pierre Cox, from the Institut d’Astrophysique de Paris and lead author of the study. “This halo weighs several trillion times the mass of our Sun.”

Dark matter makes up about 80% of all matter in the universe, but it does not emit or absorb light. Astronomers can only detect it through its gravitational effects. The HerS-3 Einstein Cross offers a unique laboratory for studying how dark matter influences the formation of galaxies in the early universe.

Because of the magnification caused by lensing, the team was able to study HerS-3 in unprecedented detail. The galaxy appears as a luminous starburst, with an inclined rotating disk and strong outflows of gas from its center. “HerS-3 formed when the universe was just two billion years old, during the peak of cosmic star formation,” says Hugo Messias, co-author of the study and astronomer at the ALMA Observatory. “Thanks to this natural telescope, we can zoom into regions 10 times smaller than the Milky Way, almost 12 billion light-years away, and in the process infer hidden matter in the light-of-sight.”

This is the first detection of an Einstein Cross at submillimeter and radio wavelengths—a milestone for facilities like ALMA that probe the cold gas and dust fueling the birth of stars in galaxies in the early universe.