A black hole as light as a truck: how dark matter could grow one inside a star

A black hole weighing only about 40 tonnes, roughly the mass of a loaded truck, sounds impossible. A black hole that tiny should vanish almost at once, evaporated by Hawking radiation. Yet a new theoretical study argues that, with a little help from dark matter, a black hole this small could not only survive but grow inside a star, slowly turning it into a black hole from within. The work, by H. A. Adarsha, Chandrachur Chakraborty and Sudip Bhattacharyya, appears in Physical Review D.

A parasite born of dark matter

Certain hypothetical dark matter particles, ultraheavy and «asymmetric,» meaning they do not annihilate when they meet, can be captured by dense stars such as neutron stars and white dwarfs. Over time they sink to the core, pile up, become self-gravitating and collapse into a minuscule black hole sitting at the star’s center. The authors call it an endoparasitic black hole: a black hole living inside its host.

A race between eating and evaporating

Once born, the tiny black hole faces a tug of war. On one side, it grows by swallowing the surrounding stellar matter and by continuing to feed on captured dark matter. On the other, Hawking radiation, the quantum process by which black holes slowly leak energy, eats away at its mass. Which side wins depends on the numbers. There are three outcomes: the hole grows without limit and eventually transmutes the whole star into a black hole; growth and evaporation balance, leaving it at a roughly constant mass; or evaporation wins and the hole disappears. The paper’s headline result is where the balance tips. The calculations show that an endoparasitic black hole with a mass as small as about 40 tonnes can already sit on the winning side and undergo sustained growth, a lower threshold than previously thought.

Why this matters, kept honest

A note on what is and isn’t established. This is a theoretical result that rests on a specific, unproven assumption: that ultraheavy asymmetric dark matter exists and behaves as modeled. It is not an observation of such a black hole. What makes it powerful is the reverse logic. If these dark-matter-fed black holes formed easily, then old, dense stars should keep quietly collapsing into black holes, and yet we still see ancient millisecond pulsars and white dwarfs that are billions of years old. Their very survival becomes evidence. By demanding that these stars last long enough, more than a billion years for pulsars and more than ten billion for white dwarfs, the authors turn them into natural dark matter detectors, tightening the limits on the particle’s mass and on how strongly it interacts with ordinary matter.

Not the black holes you were expecting

It also reframes where tiny black holes can come from. A primordial black hole, one born in the Big Bang, would need a mass above roughly a trillion kilograms to have avoided evaporating by now. The black holes in this study break that rule, because they do not have to be ancient: they can form late, deep inside a star, fed by dark matter that only recently arrived. A black hole no heavier than a truck, quietly growing at the heart of a dead star, is the kind of possibility that shows how much the nature of dark matter could still surprise us.

© 2026 Homer Dávila Gutiérrez, FRAS — SKYCR.ORG. All rights reserved. Publication: H. A. Adarsha, Chandrachur Chakraborty & Sudip Bhattacharyya, Transmutation timescales for the dark matter induced collapse of compact stars into black holes, Physical Review D 114, 063007 (2026). DOI: 10.1103/zb1m-762n