Astronomers think that galaxies cannot form without the gravitational pull of dark matter. So a trail of galaxies free of this mysterious material, with no obvious cause, would be a remarkable find. In a paper published in Nature on 18 May1, astronomers say they might have observed such a system — a line of 11 galaxies that don’t contain any dark matter, which could all have been created in the same ancient collision. But many of their peers are unconvinced that the claim is much more than a hypothesis.

This kind of system could be used to learn about how galaxies form, and about the nature of dark matter itself. “If proven right, this could certainly be exciting for galaxy formation. However, the jury is still out,” says Chervin Laporte, an astronomer at the University of Barcelona in Spain.

The finding centres on two galaxies described by Pieter van Dokkum at Yale University in New Haven, Connecticut, and his co-authors in 2018 and 2019. Their stars moved so slowly that the pull of dark matter was not needed to explain their orbits, so the team concluded that the galaxies contained no dark matter. The finding was controversial because the galaxies, named DF2 and DF4, seemed stable and different from the only other known dark-matter-free galaxies, which are new and short-lived, created in the arms of larger galaxies whose dark matter is being stripped by a neighbour. How DF2 and DF4 formed was a mystery.

Telltale trail

In the latest paper, van Dokkum’s team not only connects the two unusual galaxies, but says their properties are consistent with them being formed in a high-speed collision, eight billion years ago, that also spawned more such structures. “This single explanation explains so many odd things about these galaxies,” says van Dokkum.

The team borrowed its scenario from simulations originally created to explain unique features in larger-scale collisions between galaxy clusters. The researchers suggest that when two progenitor galaxies collided head on, their dark matter and stars would have sailed past each other; the dark matter would not have interacted, and the stars would have been too far apart to collide. But as the dark matter and stars sped on, gas in the space between the two galaxies’ stars would have crashed together, compacted and slowed down, leaving a trail of matter that later formed new galaxies with no dark matter.

Next, the researchers looked for such galaxies in the line between DF2 and DF4. They identified between three and seven new candidates for dark-matter-free galaxies, as well as strange, faint galaxies at either end, which could be the dark matter and stars remaining from the progenitor galaxies. “It was staring you in the face once you knew what to look for,” says van Dokkum.