A fluffy ball of invisible web envelops a black hole. This web is dark matter, which doesn't shine but, through its weight, governs the motion of stars on the outskirts of galaxies. Astrophysicists have long seen such 'fluffy' clouds around cosmic objects and have mapped their profiles — how density falls from the center to the edge. Now these profiles have been fitted to the black hole itself, and it turns out they don't just hang outside but grow inward, changing its very core.
In the new work, researchers took ready-made descriptions of dark halos (such as the Einasto, Burkert, or pseudo-isothermal families) and plugged them into the equations of gravity. To prevent the matter inside from collapsing into a point of infinite density, they had to replace ordinary light with a special medium — nonlinear electrodynamics. In this framework, fields interact with themselves, as if light rays slightly cling to each other, creating an elastic cushion. This cushion prevents space from compressing to zero.
The most straightforward path was for the magnetic field: knowing the halo's density profile, scientists immediately wrote down the law of the medium that produces it. The electric field required more effort — the solution is derived step by step from the equations, as if you were assembling a mechanism piece by piece without seeing the full blueprint. The main result: for all halos with finite central density, the black hole automatically acquires a smooth core and, at large distances, behaves like the ordinary one predicted by Schwarzschild. This ties together two worlds, the observed astronomy of dark halos and the theory of regular black holes, into a unified picture.
The dark halo profile is often described by a law resembling a layered cake: in the middle, density is almost constant, while toward the edges it falls off according to a strict mathematical recipe, like a sand pile being blown by the wind.
In Vernor Vinge's novel 'A Fire Upon the Deep,' galactic zones with different physics echo the idea that the properties of space inside a black hole might depend on the surrounding dark matter.