A pair of studies released in 2025 by physicist Stefano Profumo from the University of California, Santa Cruz offers two radically different yet deeply connected models for how dark matter may have formed in the early universe. Instead of relying on exotic particle candidates that have eluded direct detection, these models trace the origin of dark matter back to known physics: quantum fields, gravitational collapse, and the behavior of space itself during the universe’s earliest moments.
The first theory focuses on a gravitational process during a post-inflationary accelerated expansion period. This was not the inflation event itself, but a later, less extreme phase when the universe was still expanding faster than either radiation or matter would permit. According to Profumo’s calculations, this period may have produced stable particles through the behavior of the cosmic horizon, the boundary beyond which no information can be received due to the universe’s expansion. These particles, produced entirely through gravitational processes, would have remained in place ever since, forming the invisible scaffold known as dark matter.
The second theory turns attention toward a hidden sector of physics, one that mirrors our known universe but is fundamentally disconnected from it except through gravity. In this hidden sector, the strong nuclear force is reproduced as a separate “dark QCD” system. Instead of forming ordinary protons and neutrons, the dark quarks would form heavy composite particles known as dark baryons. Profumo argues that under specific conditions, these particles would be so massive and dense that they would collapse into stable, microscopic black holes just a few times heavier than the Planck mass. These black hole like remnants, invisible to detectors but significant in gravitational terms, could comprise all the dark matter observed in the universe today.
Both ideas sidestep the need for weakly interacting massive particles, which have failed to show up in decades of searches. Instead, Profumo builds his theories around gravitational effects that operate under extreme conditions early in cosmic history. The guiding principle is that dark matter may have never interacted with the particles we know, except through the force that shaped the universe at its largest scales.
In the horizon-based scenario, the idea draws from the same physics behind Hawking radiation. Just as a black hole can radiate particles due to quantum effects near its event horizon, a rapidly expanding universe can generate particles near its cosmic horizon. The mechanism relies on a phase of expansion known as a quasi de Sitter period, defined by a specific range of equations of state. Profumo’s model assumes a brief phase dominated by a form of energy with a pressure-to-density ratio between negative one and negative one third. Under these conditions, static observers would perceive a thermal radiation spectrum, which could lead to the creation of stable particles in a range of masses.
This process does not rely on any particular interaction between dark matter and the rest of the universe. It only requires that the particles are stable and produced in sufficient abundance. Profumo’s calculations show that, depending on the temperature and duration of this expansion phase, the resulting particle mass could vary widely from a few kiloelectronvolts up to nearly the Planck scale. This makes the mechanism flexible but also extremely difficult to test directly. No trace would remain in particle detectors. The evidence would exist only in how the universe behaves on large scales today.
The second theory deals with a very different formation pathway but arrives at a similar destination. It proposes that dark matter may be the result of black holes formed from the collapse of heavy dark baryons in a parallel version of QCD. In this hidden sector, dark quarks would be bound by a confining force analogous to the strong interaction, forming stable particles whose only connection to ordinary matter would be gravity.
What distinguishes these dark baryons from typical dark matter candidates is their potential to undergo gravitational collapse almost immediately after their formation. According to Profumo, when certain thresholds are met including specific relationships between the number of dark colors N, the confinement scale, and the mass of dark quarks the baryons can collapse into black holes with masses a few times that of the Planck scale. Once formed, these objects would persist indefinitely, assuming that Hawking evaporation does not destroy them.
To avoid early evaporation, Profumo assumes that once these black holes reach Planck-scale dimensions, quantum gravity effects dominate and prevent further mass loss. He cites multiple theoretical arguments and calculations suggesting that radiation halts or becomes negligible when the Schwarzschild radius approaches the Planck length. This assumption is not universally accepted, but it is consistent with several proposals that posit the existence of Planck-scale relics. If correct, these relics would be entirely stable and effectively undetectable by current instruments.
A key feature of the dark baryon model is that the black holes must be produced in large numbers during or just after a confinement phase transition in the hidden sector. This transition must occur before the onset of big bang nucleosynthesis to avoid disrupting the formation of light elements. Profumo calculates that the parameter space required for this to happen is large but constrained. The black holes must form early, the dark sector must cool rapidly, and the dark plasma must vanish before interfering with observable physics.
The mass of these black hole remnants scales with N, the number of colors in the dark SU(N) gauge theory. Profumo finds that viable values range from N = 10 to N = 100, depending on the details of the confinement dynamics and the temperature ratio between the dark sector and the visible sector. The upper bound is dictated by quantum gravity constraints, specifically the requirement that the number of dark gluons does not induce unacceptably large corrections to Newton’s constant through renormalization effects.
The theory also assumes that the dark sector is cooler than the visible sector, a condition necessary to prevent excessive dark radiation. Profumo demonstrates that even with this temperature gap, the production of dark baryons and their subsequent collapse into stable black holes can match the observed abundance of dark matter in the universe today.
Each of these models answers the dark matter question without invoking new particles that interact with the electromagnetic, strong, or weak forces. That makes them difficult to falsify through laboratory experiments but also robust against experimental null results. They shift the question away from what kind of particle dark matter is and toward when and how it emerged from the fabric of the universe itself.
This approach also reflects a broader trend in theoretical physics. As experimental searches fail to turn up dark matter in collider experiments, detectors, or axion searches, theorists are increasingly turning to cosmology and gravity for answers. If Profumo’s models are correct, then the key to understanding dark matter lies not in colliders or underground labs, but in the early universe where quantum effects at cosmic horizons or the dynamics of a hidden sector set the stage for a dark component that has shaped the cosmos ever since.
UC Santa Cruz has long been a hub for this kind of interdisciplinary work. The university’s history in theoretical physics includes early contributions to QCD, axion theory, and the development of cosmological models that combine particle physics with large-scale structure. Profumo’s recent publications continue that tradition, offering detailed, testable frameworks that bridge the smallest and largest scales in physics.
Whether dark matter emerged from horizon-scale radiation or from the gravitational collapse of exotic particles in a shadow universe, both ideas present an important shift. They avoid the need for direct interaction between dark matter and standard particles. They rely only on gravity, quantum field theory, and the well-understood properties of accelerated expansion. And they show that even in a universe where much remains invisible, the rules of physics still apply and may be sufficient to explain the invisible matter that holds galaxies together.
Source:
https://phys.org/news/2025-08-theories-dark-mirror-world-universe.html?
Citations:
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Profumo, S. Dark matter from quasi–de Sitter horizons. Physical Review D, 112, 023511 (2025). https://doi.org/10.1103/vmw2-4k77
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Profumo, S. Dark baryon black holes. Physical Review D, 111, 095010 (2025). https://doi.org/10.1103/PhysRevD.111.095010
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Peña, M. Theories on dark matter’s origins point to ‘mirror world’ and universe’s edge, University of California – Santa Cruz, via Phys.org (August 4, 2025). https://phys.org/news/2025-08-theories-dark-mirror-world-universe.html
