A giant cloud of invisible matter acts like a funhouse mirror — distorting the light from a distant galaxy. From these distortions, astronomers can weigh the cloud and peer into its depths. That's how, using a natural gravitational lens with the code JVAS B1938+666, scientists stumbled upon a peculiarity: hidden inside the lens is an object with the mass of a million suns, but compressed into a surprisingly small volume. Like a tiny lead shot inside a fluffy ball of cotton.
The authors of the study checked whether such a core could form from ordinary dark matter. It turned out that it cannot — unless you assume that dark matter particles sometimes collide and exchange energy, like billiard balls. Then the cloud starts to "heat up" from within, and its central part compresses even further — creating a secondary dense core, exactly like the one seen in the lens. This process is called deep core collapse, and it is naturally predicted by models of self-interacting dark matter.
Another possibility is that an intermediate-mass black hole could have flown into an ordinary cold cloud and packed the matter around it. But to match the observations, the black hole would have had to first grow in a dwarf satellite galaxy, and then lose almost all its mass — a hundred thousand times!—when the main lens galaxy stripped it via tidal forces. Whether that could happen in the real cosmos is unclear. But self-interacting dark matter handles the task without any contortions.
Gravitational lenses work like giant natural telescopes: a massive foreground object bends and magnifies the light from a distant source, allowing us to see details that no instrument could otherwise reveal.
If dark matter can indeed collide, then invisible "stars" made of dark matter could exist at the centers of galaxies — objects that emit no light but reveal themselves through gravity. This idea is explored in Stephen Baxter's novel 'Raft'.