Supermassive bodies warp space, turning into a lens that bends light from distant objects. Such spacetime curvature allows astronomers to see the invisible. Recently, this method helped detect a hidden clump with the mass of a million Suns.
Analysis of light distortions revealed the strange structure of the find. A tiny dense core — most likely a black hole —is surrounded by a vast cloud stretching hundreds of light-years with nearly constant density. In the standard model of dark matter, density should increase towards the center, like thickening jelly. Here it's the opposite, like a lens with a blurred focus. This structure aligns with the self-interacting dark matter hypothesis: its particles can collide and stick together, preventing a dense core from forming.
Einstein predicted gravitational lenses back in 1912, but thought the effect would be unnoticeable. Today, it's our main tool for 'weighing' the invisible.