The Big Bang’s Biggest Mystery? Dark Matter May Have Been “Red Hot” at Birth

Dark matter, the invisible substance that shapes the Universe, may have had a far more dramatic beginning than scientists once believed.

A research team from the University of Minnesota Twin Cities and Université Paris-Saclay is questioning a theory about dark matter that has shaped cosmology for decades. Their new work suggests that this mysterious substance may have been “incredibly hot” – moving at nearly the speed of light – when it first formed in the early Universe.

The findings were published in Physical Review Letters, the leading journal of the American Physical Society. The study offers fresh insight into how the Universe began and expands the range of possibilities for what dark matter is and how it behaves alongside ordinary matter.

Why Dark Matter Was Thought to Be Cold

For many years, scientists believed dark matter had to be cold, meaning slow-moving, when it separated from the intense radiation that filled the young Universe. This separation process is known as freezing out. The assumption was based on the idea that fast-moving particles would prevent galaxies and other large structures from forming. To test this belief, the researchers examined how dark matter might have formed during a critical early period called post inflationary reheating.

This era followed cosmic inflation, when the Universe rapidly filled with energy and particles. The team explored how dark matter created during this time could evolve as the Universe expanded and cooled.

Lessons From Neutrinos and Early Theories

“The simplest dark matter candidate (a low mass neutrino) was ruled out over 40 years ago since it would have wiped out galactic-sized structures instead of seeding them,” said Keith Olive, professor in the School of Physics and Astronomy. “The neutrino became the prime example of hot dark matter, where structure formation relies on cold dark matter. 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.”

In earlier models, neutrinos were rejected as dark matter candidates because their high speeds would have smoothed out matter instead of allowing galaxies to grow. This led researchers to strongly favor cold dark matter for decades.

How Hot Dark Matter Could Still Form Galaxies

The new study shows that dark matter does not need to be slow-moving at birth. The researchers found that dark matter particles could separate from other matter while still ultrarelativistic, or very hot, and still cool enough before galaxies began to take shape. This outcome is possible because the particles form during reheating, which gives them enough time to lose energy as the Universe expands.

“Dark matter is famously enigmatic. One of the few things we know about it is that it needs to be cold,” said Stephen Henrich, graduate student in the School of Physics and Astronomy and lead author of the paper. “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 has time to cool down before galaxies begin to form.”

Searching for Evidence and Looking Back in Time

The team plans to continue this work by identifying the most promising ways to detect these dark matter particles. Possible approaches include direct searches using particle colliders or scattering experiments, as well as indirect methods that rely on observations of the cosmos.

“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, professor from the Université Paris-Saclay in France and co-author on the paper.

Reference: “Ultrarelativistic Freeze-Out: A Bridge from WIMPs to FIMPs” by Stephen E. Henrich, Yann Mambrini and Keith A. Olive, 24 November 2025, Physical Review Letters.
DOI: 10.1103/zk9k-nbpj

This research was supported by funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement.

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