At the center of a galaxy, gravity condenses dark matter into a dense 'spike.' Such a dark spike resembles a funnel of invisible particles around a massive body. But if both the center and the particles are primordial black holes, the picture breaks down. Light holes, the size of an asteroid, are born with almost no spin. Without spin, they can't stay in orbit and fall onto the central giant.
The authors traced which forces could spin up the light black holes and save them from falling. They considered kicks from large clumps of matter, the influence of neighbors, and other perturbations. Calculations and simulations showed that in the very core, where density should be maximal, no mechanism provides sufficient angular momentum.
As a result, the inner region of the spike turns out to be nearly empty — much sparser than in particle models. This is not just a theoretical nuance. If dark matter consists of primordial black holes, the signatures of their annihilation or gravitational waves will be weaker than expected. But the emptiness itself at the center could become a clue, a kind of shadow revealed by gravitational echo.
An asteroid-mass black hole with the mass of Mount Everest would fit inside an atomic nucleus, so strongly is the matter compressed.
In Arthur C. Clarke's novel 'The Dark Light', an invisible cloud of black holes threatens the stars, but those were much larger.