A new theoretical study proposes that primordial black holes—hypothetical objects believed to have formed in the early universe—may exist in a hidden dimension of space, often referred to as a dark fifth dimension. This dimension, which is estimated to be about one micron wide, is described as invisible and influenced by gravity. It could allow extremely small primordial black holes to exist without being directly detectable. Unlike black holes formed from the collapse of stars, primordial black holes are thought to have formed from dense regions of the early universe. They have long been considered potential candidates for dark matter, an invisible form of matter that makes up a large portion of the universe's mass. Most traditional studies consider primordial black holes as four-dimensional objects, fitting the standard cosmological model that describes the universe as having three spatial dimensions and one time dimension. However, recent research suggests that these black holes should be re-evaluated, considering the possibility of an additional dark spatial dimension. In this scenario, while matter and regular forces are restricted to the familiar four dimensions, gravity could extend into the hidden dimension, affecting the formation and survival of primordial black holes. A study led by Luis A. Anchordoqui from Lehman College, Alek Bedroya from Princeton University, and Dieter Lüst from the Max-Planck-Institut für Physik and LMU Munich explores the hypothesis of a fifth dimension using constraints from quantum gravity. In their detailed study published in Physical Review D, the researchers re-examine the formation mechanisms of primordial black holes (PBH) and ask whether, in the dark dimension scenario, PBH are indeed four- or five-dimensional black holes. Applying quantum gravity constraints, they show that any viable mechanism, except for the existence of exotic new physics at low energies, inevitably leads to the formation of five-dimensional primordial black holes. The researchers also analyze the lifetime of these five-dimensional primordial black holes, exploring the possibility that some of them might evaporate late in their existence. In the scenario they studied, the constraints they impose suggest that some primordial black holes would initially behave as classical four-dimensional objects before becoming unstable and transforming into five-dimensional objects. According to their analysis, primordial black holes formed from the collapse of cosmic string loops would directly be five-dimensional structures. Cosmic strings are hypothetical topological defects that could form during phase transitions in the early universe. The additional dimension would influence the potential lifespan of primordial black holes. The researchers suggest that primordial black holes formed from cosmic strings could live for a time comparable to the age of the universe. However, as this is a hypothesis, further research and observational evidence are needed to support it.