Dark matter is one of the biggest mysteries in science.
We know it exists—because it makes up about 80% of all matter in the universe and helps hold galaxies together—but no one knows exactly what it is made of or where it came from.
Two new studies by Professor Stefano Profumo, a physicist at the University of California, Santa Cruz, explore bold new ideas about its origins, suggesting that dark matter could come from a hidden “mirror world” or even from the universe’s own horizon.
The evidence for dark matter is overwhelming.
It explains why galaxies rotate the way they do, matches observations of the cosmic microwave background, and fits our understanding of the universe’s large-scale structure.
But despite decades of searching, scientists still haven’t detected a dark matter particle directly. Profumo’s new work focuses on how dark matter could have formed naturally in the early universe, without relying on the idea that it must be an exotic particle we can easily detect.
His most recent paper, published in Physical Review D in July, looks at the possibility that dark matter formed in a hidden “dark sector” of the universe—a kind of mirror world with its own particles and forces. This shadowy realm would be invisible to us, but it could follow similar physical laws.
In this idea, the strong nuclear force that binds quarks together inside protons and neutrons in our world would also exist in the dark sector as “dark QCD,” binding together dark quarks and dark gluons into heavy particles called dark baryons.
In the early universe, these dark baryons could have been so dense that they collapsed under their own gravity into tiny, stable black hole–like objects. These would be only a few times heavier than the Planck mass—the basic mass scale in quantum gravity—but they could make up all the dark matter we see today.
Because they would interact only through gravity, they would be completely invisible to experiments, yet they would still influence how galaxies and other cosmic structures form.
Profumo’s other recent paper, published in May, explores a different idea: that dark matter might have been created by the universe’s own “cosmic horizon,” similar to a black hole’s event horizon. This horizon marks the limit of what we can see in the expanding universe.
The study suggests that if the early universe experienced a brief period of accelerated expansion after the inflation era—less extreme than inflation but faster than normal—this expansion could have created particles through quantum effects.
In this scenario, dark matter would be generated purely through gravity, without needing to interact with normal matter at all.
Both ideas are speculative but grounded in well-known physics, such as quantum field theory in curved spacetime and the properties of gauge theories. They also avoid depending on conventional dark matter models, which have faced increasing challenges as experiments continue to find nothing.
Profumo, who literally wrote the textbook on particle dark matter in 2017, sees these theories as part of a long tradition at UC Santa Cruz of connecting deep questions in particle physics with the large-scale behavior of the cosmos.
By looking at both hidden worlds and the universe’s own edges, his work keeps open the possibility that the answer to dark matter’s origins may be stranger—and more fascinating—than we’ve ever imagined.
