A new study by astronomers at the University of Minnesota challenges one of the central dogmas of modern cosmology: that dark matter must be born cold. The researchers show that dark matter could have been created extremely hot, moving at nearly the speed of light, and still cooled down in time to form galaxies. The work was published in Physical Review Letters.

Why Dark Matter Is Assumed to Be Cold

Dark matter makes up about 85 percent of all matter in the Universe and outweighs ordinary baryonic matter by roughly a factor of five. It does not interact with electromagnetic radiation, which is why it is invisible and neither emits, absorbs, nor reflects light. This makes it extremely difficult to detect.

The most widely accepted model, Lambda-CDM, assumes that dark matter is cold, meaning its particles move much more slowly than the speed of light. This allows them to form gravitational clumps around which ordinary matter later gathers, leading to the formation of galaxies.

Hot dark matter, whose particles move close to the speed of light, has long been considered impossible because it would spread out too quickly and prevent the formation of structures on galactic scales. A classic example is low-mass neutrinos, which were ruled out as dark matter candidates in the 1980s precisely for this reason.

A New Origin: Post-Inflationary Reheating

The authors propose an alternative scenario. Dark matter could have been born extremely hot during post-inflationary reheating, the phase immediately after cosmic inflation when the inflaton field decayed and transformed into a hot, dense soup of radiation and particles.

The team explains that dark matter is famous for its mystery, and one of the few things scientists thought they knew for certain was that it had to be cold. For the past four decades, most researchers therefore assumed that dark matter was born cold in the early Universe. Their new results show that this does not have to be the case: dark matter could have been born extremely hot and still cooled down before galaxies began to form.

The key concept is decoupling. Dark matter stops interacting strongly with ordinary matter and radiation while it is still very hot. After that, it has enough time to slow down and start behaving like cold dark matter, allowing it to drive the formation of the first galaxies.

The Return of an Old Candidate

This scenario revives one of the earliest and simplest candidates: low-mass neutrinos. They were long considered an example of hot dark matter and were rejected because they would suppress structure formation. Now, the researchers show that if neutrinos were produced at the right moment during post-inflationary reheating, they would have had time to cool and effectively become cold.

The authors note that neutrinos became the textbook example of hot dark matter, in contrast to the cold dark matter required for structure formation. They find it remarkable that a similar candidate, if created precisely during the emergence of the hot Big Bang, could cool enough to behave like cold dark matter.

What Comes Next

The team plans to search for such particles on Earth through accelerator experiments and through indirect observations of the early Universe. If confirmed, this hypothesis could not only reveal the true nature of dark matter but also shed light on one of the most mysterious epochs in cosmology: post-inflationary reheating.

In Brief

Scientists have shown that dark matter could have been born hot, moving at nearly the speed of light, during post-inflationary reheating and still cooled down to the cold state required for galaxy formation. This revives low-mass neutrinos as a viable candidate and may require revisions to the standard Lambda-CDM model. The discovery opens new pathways for searching for dark matter particles and for studying the earliest stages of the Universe.