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As early as Vera Rubin found that stars on the outskirts of galaxies move too fast — as if held by an invisible web. And Fritz Zwicky was the first to suspect that clusters of galaxies hide enormous invisible mass. Today we call it dark matter and try to understand what it's made of.

New simulations have revealed: if dark matter particles are capable of sticking together, they clump into dense blobs. A blob with the mass of a million Suns disturbs the stellar stream GD-1 — a long thread of torn-apart stars — like a speck stuck in a web. And the same blob exactly matches the properties of an invisible object found through a gravitational lens.

Moreover, sticky blobs of dark matter naturally gather stars around themselves. That's probably how the mysterious Fornax 6 cluster in the Fornax dwarf galaxy came to be.

And here's the twist: the same mechanism tied together three puzzles at once. And the GD-1 stream itself is so huge that it occupies more than a third of the celestial hemisphere — the invisible framework of the cosmos reveals itself on unexpected scales.

Original abstract

as written by the authors arXiv:2510.11006

We show that core-collapsed self-interacting dark matter halos of mass $$\sim 10^6\,{M_\odot}$$, originally simulated to explain the dense perturber of the GD-1 stellar stream, also reproduce the structural properties inferred for the dense perturber detected in the strong lensing system JVAS B1938+666 from radio observations. Furthermore, these halos are sufficiently compact and dense to gravitationally capture field stars in satellite galaxies of the Milky Way, providing a natural explanation for the origin of Fornax 6, a stellar cluster in the Fornax dwarf spheroidal galaxy. Our results demonstrate that observations of halos with similar masses but residing in different cosmic environments offer a powerful and complementary probe of self-interacting dark matter.