A tiny black hole the size of an atomic nucleus floats in the hot soup of the early universe. Normally, such a little one should quickly evaporate, releasing energy like a drop of water on a hot skillet. But if space is more complex than it seems and contains hidden dimensions, everything changes.
This study explores a universe where, in addition to the usual three dimensions, there are extra ones curled up into tiny rings. In such a geometry, a black hole of the same mass has a larger horizon: its "surface" is wider and its temperature is lower. So it evaporates more slowly than its four-dimensional sister. Meanwhile, the radiation around it hasn't gone anywhere: photons and particles fall onto the hole like snow on hot asphalt, and it begins to grow. This process becomes avalanche-like: the more massive the hole, the stronger it attracts matter, and growth accelerates.
The authors calculated this scenario and found that if, at the birth of the universe, very few such holes appeared, about one per 10⁴⁴ ordinary particles, then by the time matter and radiation became equal in influence, they could have grown to stellar masses. That's exactly how much dark matter we observe today. Previously, it was thought that a much larger initial fraction of holes would be needed, which didn't fit observations well. Extra dimensions remove this contradiction: holes don't have to be born as giants; they can grow in the process.
Such growth is not just a mathematical curiosity. It opens a new way to explain dark matter without invoking unknown particles. Instead of searching for invisible building blocks of the universe, we can allow that space is deeper and black holes are trickier than we thought. And then dark matter is not a mysterious substance but a scattering of grown primordial holes whose roots reach into hidden dimensions.
If our universe had two extra dimensions, a black hole with the mass of a small asteroid could grow to the mass of the Sun over a time comparable to the age of the universe.
The idea of hidden dimensions where black holes can grow to enormous sizes recalls the world of "Interstellar," where characters pass through a wormhole connected to five-dimensional space.