Dough rises faster when it already contains yeast. Similarly, galaxies may have assembled faster if they already had black holes inside. Astrophysicists considered a scenario where the universe contained primordial black holes from the very beginning — objects that could have formed in the first instants after the Big Bang. Such black holes, if massive enough, could have been part of dark matter — the invisible substance that holds galaxies together.
Usually, a galaxy grows slowly: gas gradually clumps under its own gravity, taking hundreds of millions of years. But if a black hole with a mass of one hundred million suns already sits at the center of a future galaxy, it acts like a ready-made weight that immediately attracts gas. Calculations show that a galaxy around such a "seed" could assemble in just one hundred million years — several times faster than without it.
Interestingly, faint, barely visible galaxies might be the remnants of this process. If the seed black hole was smaller, it couldn't hold on to much gas, and only a weak, diffuse galaxy grew around it. Today, such galaxies are called diffuse or low-surface-brightness galaxies: they are hard to see, but they may preserve the memory of the first black holes.
So black holes might not only destroy but also create: they could have accelerated the appearance of the first galaxies, just as yeast accelerates the rising of dough. And if we find many faint galaxies, that could be a hint that primordial black holes really do exist.
as written by the authors arXiv:2603.04784
If massive primordial black holes (PBHs) exist and constitute a fraction of the dark matter, they can dramatically catalyze galaxy formation. By acting as pre-existing, high-density seeds, they can shorten the galaxy assembly time to as little as 100 Myr for up to 10^8 solar mass PBH seeds, allowing for the rapid formation of host halos. Furthermore, low surface brightness or diffuse galaxies may represent a natural outcome of this process, perhaps as the residue of halos seeded by smaller PBHs that failed to accrete a major baryonic component.
One hundred million years is less than one percent of the universe's current age. By cosmic standards, a galaxy around a massive black hole would assemble faster than a human can blink.