Dark Matter’s Earliest Moments May Have Been Far More Extreme Than We Thought

New research suggests the cosmos’s missing matter cooled down just in time.

Dark matter makes up most of the matter in the Universe (about 85%), yet it does not emit light or interact directly with telescopes. Scientists infer its existence from its gravitational effects on galaxies and galaxy clusters. Standard cosmological models have long favored ‘cold’ dark matter, because fast-moving particles would smooth out small density fluctuations and prevent galaxies from taking shape.

Now, a new study suggests there’s more to it than meets the eye.

Physicists at the University of Minnesota Twin Cities and Université Paris-Saclay report that dark matter could have begun its existence racing near the speed of light. Yet it still ends up behaving like the slow dark matter required for galaxies to form. Their findings, published in Physical Review Letters, revisit a largely overlooked moment just after the Big Bang and reopen one of cosmology’s oldest debates.

“Dark matter is famously enigmatic. One of the few things we know about it is that it needs to be cold,” said Stephen Henrich, a graduate student at the University of Minnesota and lead author of the study.

“As a result, for the past four decades, most researchers have believed that dark matter must be cold when it is born in the primordial universe. Our recent results show that this is not the case; in fact, dark matter can be red hot when it is born but still have time to cool down before galaxies begin to form.”

The Forgotten Era

The new work focuses on a short and poorly understood period in cosmology called reheating, which came after cosmic inflation. Inflation rapidly expanded the early Universe, but when it ended, its energy did not immediately become matter. Instead, that energy slowly transformed into particles while the Universe kept expanding.

Earlier models assumed that reheating happened almost instantly, leaving no time for hot particles to cool before radiation dominated the Universe. By relaxing that assumption, the researchers found that ultrarelativistic dark matter could lose enough energy to become compatible with galaxy formation later on.

Henrich and his colleagues asked what happens if dark matter decouples earlier, during reheating itself. They call this process ultrarelativistic freeze-out. Turns out, dark matter stops interacting while still extremely hot, then cools as the Universe expands until it behaves like conventional cold dark matter.

The mechanism itself is not exotic. It is the same process by which ordinary neutrinos decoupled from the early Universe, when they stopped interacting with other particles at temperatures of about one million electron volts (roughly 10 billion degrees Celsius), while still moving at near-light speed.

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“The simplest dark matter candidate, a low mass neutrino, was ruled out over 40 years ago since it would have wiped out galactic-size structures instead of seeding it,” said Keith Olive, a professor at the University of Minnesota and a co-author of the study, in a statement.

“It is amazing that a similar candidate, if produced just as the hot Big Bang Universe was being created, could have cooled to the point where it would in fact act as cold dark matter.”

A Bridge Between Theories

The finding lands at a delicate moment in dark matter research. For years, experimental searches have focused on WIMPs—weakly interacting massive particles that naturally freeze out cold. WIMPS are considered the leading candidate for dark matter, yet despite extensive efforts, no convincing signal has appeared.

Other ideas, such as feebly interacting massive particles, avoid these limits by interacting very little with ordinary matter. That also makes them extremely hard to detect.

Ultrarelativistic freeze-out falls between these approaches. In this scenario, In this scenario, dark matter decouples early yet remains compatible with observations of galaxies and the cosmic microwave background.

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The researchers found that, under realistic conditions during reheating, dark matter particles heavier than a few thousand electron volts would slow down enough before cosmic structures began to form. That result brings back many particle candidates that scientists had previously ruled out.

Dark matter formed this way could carry information about reheating itself, a phase of the Universe that is otherwise difficult to study.

“With our new findings, we may be able to access a period in the history of the Universe very close to the Big Bang,” said Yann Mambrini, a physicist at Université Paris-Saclay and a co-author of the study.