For years, three cosmic phenomena have puzzled astronomers. An ultradense concentration of matter distorts the light from a distant galaxy, a stream of stars bears a strange scar, and a star cluster unexpectedly formed in a dwarf galaxy. What if these three enigmas, observed at vastly different scales, shared a common origin: dark matter that interacts with itself?
Dark matter makes up about 85% of the matter in the Universe, but it remains invisible because it does not interact with light. In the standard model of cosmology, it is called "cold" and behaves like ghosts: its particles pass through each other without ever colliding. This non-interacting cold dark matter explains many things, but not the three anomalies mentioned.
JVAS B1938+666: a black ring and a central point show an infrared image of a distant galaxy distorted by gravitational lensing. The orange emission represents radio waves from the same system.
Credit: Devon Powell, Max Planck Institute for Astrophysics, based on data from Keck/EVN/GBT/VLBA. The first strange case concerns the JVAS B1938+666 system, a distant galaxy whose light is distorted by a gravitational lens. Astronomers detected an abnormally dense concentration of matter there. The second is the GD-1 stellar stream, a trail of stars in our Milky Way that exhibits a "scar" — as if a massive, invisible object had plowed through it. Finally, the Fornax 6 cluster, in the Fornax dwarf galaxy, appears to have formed too quickly to be explained by cold dark matter.
One solution is to imagine dark matter that interacts with itself. Unlike the ghostly particles of the standard model, these particles can collide, exchanging energy and momentum. These collisions cause a "gravothermal collapse" that creates dense, compact cores of dark matter. Such cores could then act as gravitational traps or invisible lenses.
According to Hai-Bo Yu, a researcher at the University of California, Riverside, this mechanism works at three very different scales: in the distant Universe, in our Galaxy, and in a satellite galaxy. The densities observed in the three enigmas are difficult to reproduce with standard dark matter, but they arise naturally if dark matter interacts. The interactions allow the internal structure of dark matter halos to be reshaped, producing sufficiently high concentrations.
This research thus offers an intriguing way to unify cosmic puzzles that seemed unrelated. The results were published in the journal
Physical Review Letters in April 2026. If self-interacting dark matter is confirmed, our view of the Universe and its composition could be profoundly changed.