What if the mysterious substance holding galaxies together is made of leftovers from a universe that existed before our own? That is the central question posed by physicists Carlo Rovelli and Francesca Vidotto in a peer-reviewed paper published in 2018 in the journal Universe. Their proposal, which continues to draw attention from the quantum gravity community as of spring 2026, argues that black holes from a pre-Big Bang cosmic phase could have collapsed to unimaginably tiny remnants that survive to this day and collectively behave like dark matter.
The idea sits at the intersection of three of physics’ deepest unsolved problems: the identity of dark matter, the origin of the Big Bang, and the reason time flows in one direction. If Rovelli and Vidotto are right, a single mechanism could address all three.
The bounce and what it leaves behind
Standard cosmology treats the Big Bang as the starting point of everything. Rovelli and Vidotto work within a different framework called bouncing cosmology, built on loop quantum gravity, a leading candidate theory for merging Einstein’s general relativity with quantum mechanics. In this picture, our universe did not spring from nothing. Instead, a previous universe contracted under its own gravity, reached an almost inconceivable density, and then rebounded outward in the explosion we call the Big Bang.
During the contracting phase, matter would have clumped together and formed black holes, just as it does in our expanding universe today. But here the story diverges from the familiar one. According to loop quantum gravity, a black hole does not simply crush matter into a point of infinite density. Instead, quantum effects kick in at the smallest meaningful scale in physics, the Planck scale, roughly 10-35 meters. At that threshold, the black hole undergoes a quantum transition and becomes what theorists call a “white hole,” an object that radiates outward rather than swallowing everything around it.
Rovelli and Vidotto argue that these Planck-scale remnants, the shrunken husks of pre-Big Bang black holes, would have survived the bounce and persisted into the expanding universe we inhabit. Because they are extraordinarily small and interact almost not at all with light or ordinary matter, they would be invisible to telescopes yet still exert gravitational pull. In other words, they would look and act exactly like dark matter.
Connecting dark matter to the arrow of time
A companion paper by the same researchers, available as an arXiv preprint (a manuscript that has not undergone formal peer review in a journal), extends the argument to one of the most puzzling facts about our universe: its remarkably low entropy at birth. Entropy is a measure of disorder, and the second law of thermodynamics says it always increases. The early universe, however, was extraordinarily ordered, a condition physicists call the “past low entropy” problem. No one has a fully satisfying explanation for why the cosmos started in such a tidy state.
Rovelli and Vidotto suggest that the formation of black holes during the contracting phase effectively locked away enormous amounts of entropy. When those black holes shrank to Planck-scale remnants during the bounce, the entropy they carried became inaccessible to the rest of the universe. The result: the newly expanding cosmos began in a low-entropy state, not by coincidence or fine-tuning, but as a natural consequence of the bounce itself. A separate arXiv preprint (also not peer-reviewed in a journal) laid out the earliest version of this reasoning, providing a timestamped record of how the idea developed.
How this differs from primordial black holes
The notion that black holes could account for dark matter is not new. Primordial black holes, hypothetical objects that formed from density fluctuations in the early universe, have been studied as dark matter candidates for decades. But the Rovelli-Vidotto remnants are fundamentally different in scale and origin.
Primordial black holes in most models range from a fraction of a solar mass to thousands of solar masses. They are large enough, in principle, to be detected through gravitational lensing, the bending of light from distant stars as a massive object passes in front of them. Microlensing surveys such as OGLE have searched for exactly this signal. A 2019 analysis published in Nature Astronomy by Niikura et al. found that primordial black holes appear too rare in the mass ranges OGLE can probe to account for all of dark matter.
Rovelli and Vidotto’s remnants, by contrast, sit at the Planck scale, roughly 20 orders of magnitude lighter than the lightest primordial black holes that lensing surveys can constrain. Current observational bounds simply do not reach them. That is both the proposal’s greatest strength and its most significant weakness: nothing in existing data rules the remnants out, but nothing confirms them either.
A peer-reviewed study in Physical Review D has separately analyzed primordial black hole production in a pre-Big Bang scenario featuring an early matter-dominated era, identifying parameter ranges where such objects could form in mass windows relevant to dark matter. That work shares a philosophical ancestor with the Rovelli-Vidotto model, both assume pre-Big Bang conditions can seed dark matter, but the two differ sharply in the specific mechanisms and mass scales involved.
Why confirmation remains out of reach
No experiment or observation has confirmed the existence of Planck-scale remnants. The objects would be far too small for any existing telescope, particle collider, or gravitational-wave detector to register individually. Their collective gravitational influence would blend seamlessly into the overall dark matter density that astronomers already measure, offering no distinctive fingerprint.
The underlying framework faces its own hurdles. Loop quantum gravity, while mathematically sophisticated and actively researched, has not yet produced a prediction that has been confirmed by experiment. Bouncing cosmology remains one of several competing models for the universe’s origin, alongside standard inflationary cosmology and other alternatives. Without independent verification of the bounce itself, the remnant hypothesis rests on a chain of assumptions that, while internally consistent, are unproven.
Broader constraints on black-hole dark matter models add another layer of complexity. Review literature on primordial black hole constraints, including widely cited work by Carr, Kohri, Sendouda, and Yokoyama, has shown that limits derived from early-universe perturbations depend sensitively on modeling choices, including the assumed shape of the black hole mass function and the physics of formation. Those same reviews demonstrate that primordial black hole bounds change substantially when the objects have extended, non-monochromatic mass distributions rather than a single fixed mass. Even small changes in these assumptions can open or close entire regions of allowed parameter space. For the remnant scenario, this translates into wide theoretical uncertainty about how many such objects could exist without contradicting current data.
Where the idea stands in the dark matter landscape
Dark matter remains one of the biggest open questions in physics. The leading candidates, weakly interacting massive particles (WIMPs) and axions, have been hunted for decades without a confirmed detection. Underground detectors, particle colliders, and space-based observatories have progressively narrowed the parameter space for WIMPs, while axion searches are still ramping up. Against this backdrop, the Rovelli-Vidotto proposal adds a genuinely novel entry to the roster: dark matter not as a new particle, but as a gravitational relic of a previous cosmic epoch.
For now, the model should be understood as a speculative but serious theoretical contribution. It is grounded in detailed calculations, published in a peer-reviewed journal, and engages honestly with existing observational constraints. It does not claim to have solved the dark matter problem. What it does is demonstrate that, within the loop quantum gravity framework, pre-Big Bang black holes could naturally produce remnants with the right properties to serve as dark matter, while simultaneously offering a thermodynamic explanation for the universe’s low-entropy beginning.
Whether nature actually took this path is a question that will require tools physicists do not yet possess: probes sensitive to Planck-scale physics, independent tests of bouncing cosmology, or perhaps an entirely unexpected observation that points toward remnants no one was looking for. Until then, the work by Rovelli and Vidotto stands as a reminder of how much remains unknown about the universe’s earliest moments and the invisible scaffolding that holds it together.
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*This article was researched with the help of AI, with human editors creating the final content.
