In the vast, mysterious expanse of the universe, there’s a hidden force that holds the cosmos together, one that scientists have been chasing for decades. It’s called dark matter—a substance that, despite accounting for about 85% of the universe’s total mass, remains completely invisible and elusive. It doesn’t emit light, nor does it interact with normal matter except through gravity. And yet, without it, the universe as we know it wouldn’t exist. But until recently, the question of where dark matter came from, and what it is made of, has remained one of the most profound mysteries in physics.
A new theory, emerging from a research study led by Stephen Henrich and Keith Olive at the University of Minnesota, has just shifted the search for dark matter in a direction no one saw coming. What if the origins of dark matter are much older than we previously thought? What if, instead of emerging from one of the familiar candidates like WIMPs or FIMPs, dark matter particles could have appeared right after the Big Bang, in a way so subtle that they’ve eluded all detection until now?
A New Theory Emerges
Henrich and Olive’s team suggests a new mechanism for dark matter’s creation, one that might have occurred far earlier in the universe’s history than previous theories predicted. Their groundbreaking work proposes that dark matter could have been formed by “ultra-relativistic freeze-out” (UFO), a process that could have decoupled dark matter particles from Standard Model particles much earlier in the universe’s evolution.
The idea of ultra-relativistic freeze-out isn’t exactly new. It was first proposed as a potential explanation for dark matter decades ago, but it was largely abandoned due to a series of calculations in the 1980s that showed it wouldn’t align with the formation of galaxies. Back then, the theory was dismissed because ultra-relativistic dark matter—particles moving at nearly the speed of light—was believed to have smoothed out the variations in the universe’s density, making it impossible for the first galaxies to form.
But Henrich and Olive aren’t giving up on UFO so easily. In their study, published in Physical Review Letters, they suggest that UFO could have happened much earlier than previously thought. This new timing—shortly after the end of the Big Bang’s inflationary period—could explain why UFO dark matter has remained undetectable.
Revisiting the Early Universe
Imagine the scene shortly after the Big Bang, when the universe was a hot, dense soup of particles and energy. For a brief moment, the universe expanded exponentially during a phase known as inflation. During this time, all of the universe’s energy was contained in a single quantum field—the “inflaton.” This caused the universe to cool down dramatically and created the perfect conditions for the formation of particles and forces as we know them.
Henrich and Olive’s key insight is that ultra-relativistic freeze-out might have occurred right at the end of this inflationary period, during a phase called “reheating.” This is the moment when the inflaton decays into Standard Model radiation particles—essentially laying the groundwork for the universe’s structure. According to their calculations, if dark matter particles decoupled from these radiation particles during reheating, they would have retained a “hot” state for some time. But over the course of the radiation-dominated era that followed, these particles would have cooled, reducing their disruptive effects on early galaxy formation.
By taking this new timeline into account, Henrich’s team has managed to give UFO dark matter a fighting chance to explain both its undetectability and its compatibility with the formation of the first galaxies.
A New Candidate for Dark Matter
Henrich and his colleagues didn’t stop there. They also demonstrated that the UFO mechanism could bridge the gap between two other dark matter candidates—the WIMP (weakly interacting massive particle) and FIMP (feebly interacting massive particle)—while also opening the door to the discovery of a new, long-forgotten category of dark matter particles.
As Henrich explains, UFO dark matter would have interacted even more weakly with normal matter than WIMPs—particles that interact through the weak nuclear force and are widely studied in dark matter research. But UFO particles would not be as weakly interacting as FIMPs, which interact with Standard Model particles only in the most subtle ways. This range of interaction strengths opens up a whole new avenue for testing and detection.
This discovery is not just theoretical; it carries real-world implications for how we might detect dark matter in the future. The team’s findings suggest that the masses and interaction strengths of UFO dark matter particles could align with current or upcoming dark matter detection experiments. In other words, what was once thought to be a neglected, unlikely theory may now be a promising candidate for future detection.
“Since we determined which DM masses and interaction strengths are compatible with this mechanism, it may be possible for current or future dark matter detection experiments to find this long-neglected category of dark matter candidates,” Henrich notes. This means that the hunt for dark matter could be far from over. In fact, it might have just taken a crucial step forward.
Why This Matters
This new theory is more than just an interesting twist in the search for dark matter. It challenges our entire approach to understanding the early universe. For decades, researchers have been fixated on the idea that dark matter must have formed in the same way as the more familiar particles we know—particles that were part of the Standard Model. We’ve spent years searching for elusive WIMPs and FIMPs, but as Henrich points out, their detection has proven difficult. If UFO dark matter particles are out there, we could be looking in the wrong places.
By rethinking the early moments of the universe, Henrich and Olive’s team have not only revived a forgotten theory but have given scientists new tools and frameworks for approaching the dark matter problem. Their work underscores a central truth in science: sometimes the answers we’re seeking aren’t just waiting to be discovered—they’re waiting to be reinterpreted.
And so, as scientists continue to search for the elusive particles that make up dark matter, the question remains: could UFO dark matter be the key to unraveling one of the universe’s deepest secrets? Only time—and the right experiments—will tell.
More information: Stephen E. Henrich et al, Ultrarelativistic Freeze-Out: A Bridge from WIMPs to FIMPs, Physical Review Letters (2025). DOI: 10.1103/zk9k-nbpj. On arXiv: DOI: 10.48550/arxiv.2511.02117
