Is dark matter ‘natural?’ A UC Santa Cruz physicist puts the question to the test
A quantitative study of different dark-matter scenarios finds that primordial black holes can be just as ‘natural’ as traditional particle theories
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UC Santa Cruz Physics Professor Stefano Profumo
view moreCredit: Photo by Carolyn Lagattuta, UC Santa Cruz
SANTA CRUZ, Calif. – Dark matter is one of the oldest open problems in physics. It makes up most of the matter in the universe and shapes how galaxies form and move, yet no one has ever directly detected it. Physicists have proposed dozens of candidates to explain it: exotic new particles, black holes formed moments after the Big Bang, and more.
Returning to ‘naturalness’
With no direct detection to settle the question, researchers often fall back on a different test: not “is this candidate detected,” but “is this candidate natural.” Naturalness is physics’ version of Occam’s razor. A theory is called natural if it explains the universe we see without requiring its underlying numbers to be delicately, almost implausibly, fine-tuned. A theory that only works if several unrelated quantities happen to cancel out to many decimal places is treated with suspicion, even if it isn’t strictly ruled out.
Naturalness has quietly guided decades of research into which ideas are worth pursuing, but it’s a slippery concept: intuitive to invoke, hard to pin down, and rarely applied evenhandedly across competing ideas.
That’s the gap Stefano Profumo, professor of physics at the University of California, Santa Cruz, addresses in a new paper published in Physical Review D. Rather than debating naturalness in the abstract, Profumo puts it to a direct, quantitative test, applying the same yardstick to two very different dark matter candidates: subatomic particles, and primordial black holes—the latter being hypothetical phenomena forged in the first fraction of a second after the Big Bang, long before stars existed to collapse into them.
Measuring fine-tuning across models
That yardstick, called the Barbieri-Giudice measure, asks a simple question of any model: If you nudge one of its input numbers by a tiny amount, how much does the predicted outcome swing? A gentle swing means the model is forgiving of its own assumptions, and hence, natural. A wild swing means the model only works because its numbers have been tuned within a hair’s breadth of what’s required.
Profumo ran 12 well-studied dark matter scenarios through this test, including several flavors of particle dark matter—like the long-favored “WIMP,” or weakly interacting massive particle—and several distinct ways primordial black holes could have formed in the early universe. The result cuts against a common assumption in the field.
Dark matter candidates are often labeled “natural” or “fine-tuned” as entire categories, but this comparison tells a more nuanced story. Each bar shows how sensitively the predicted amount of dark matter responds to small changes in a model’s underlying parameters; longer bars indicate that more precise adjustment is required. Particle candidates and primordial-black-hole scenarios appear across the spectrum, showing that fine-tuning depends primarily on how dark matter is produced—not simply on what dark matter is (graphic created with OpenAI Codex, based on Fig. 6 in Profumo’s new paper).
“There’s a habit of treating primordial black holes as the exotic, fine-tuned alternative, and particle dark matter as the safe, natural default,” said Profumo, deputy director for theory at the Santa Cruz Institute for Particle Physics. “When you actually run the numbers side by side, that story doesn’t hold up. Some black hole scenarios are about as natural as it gets. Some particle scenarios are wildly fine-tuned. And some of each land right in the middle.”
For instance, black holes formed from collapsing networks of structures called “domain walls” came out among the most natural constructions in the entire study, rivaling the most forgiving particle models. Meanwhile, one of the most popular particle scenarios, in which dark matter annihilates through a resonance tied to the Higgs boson, turned out to be among the most fine-tuned scenarios Profumo examined, requiring one of its numbers to be pinned down to within a fraction of a percent. Other constructions, on both sides, fell somewhere in between.
Challenging common assumptions
“Naturalness has real power as a filter for deciding where to look next,” Profumo said. “But it can’t be a shortcut for dismissing an entire category of ideas, like primordial black holes, just because a few individual models within that category happen to be tuned. The tuning lives in the specific model, not in the kind of dark matter you started with.”
The paper doesn’t identify a winner. Instead, it offers researchers a common ruler for comparing vastly different dark matter proposals on equal footing—and a caution against letting a single word like “natural” do more work than it’s earned.
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Journal
Physical Review
Method of Research
Systematic review
Subject of Research
Not applicable
Article Title
Primordial black hole dark matter: A quantitative parameter sensitivity comparison across formation mechanisms and particle candidates
Article Publication Date
10-Sep-2026
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