For decades, scientists have been locked in a cosmic game of hide-and-seek with dark matter, the invisible substance that makes up 85% of the universe’s mass. New research suggests that the rules of this game might be entirely wrong. The standard model assumes dark matter is “cold,” meaning its particles move slowly. But a growing body of evidence hints that it might have been born “hot,” moving at near-light speeds. If true, this discovery could overturn established theories about how galaxies formed and reshape our understanding of the universe’s early moments.
The Dark Matter Puzzle
Dark matter is notoriously difficult to study because it doesn’t interact with light. This means we can’t directly see it, only infer its existence through its gravitational effects on visible matter. This also makes it impossible to be composed of the same stuff as everyday objects (electrons, protons, neutrons), which all interact with light. Despite making up five times more matter than everything we can see, dark matter remains a mystery.
The leading theory has long favored cold dark matter (CDM), particles moving slowly enough to clump together and seed the formation of galaxies. But this model isn’t perfect; certain observations challenge its predictions. That’s where the “hot” dark matter hypothesis comes in.
A Hotter Origin Story
The new research proposes that dark matter may have originated in the immediate aftermath of the Big Bang, during a period called post-inflationary reheating. This was a chaotic era when the universe rapidly expanded, converting energy into a hot, dense soup of particles. If dark matter was born in this environment, it could have been incredibly fast-moving.
The key is that even at these speeds, dark matter could have cooled down enough to behave like the “cold” version we expect. This process, called “decoupling,” would allow it to eventually clump together and drive the formation of galaxies.
Reviving Old Candidates
This discovery could also revive a long-dismissed dark matter candidate: the low-mass neutrino. Decades ago, these particles were ruled out because they were thought to move too fast to allow galaxies to form. But if they were born hot and then cooled down, they could have played a crucial role in structure formation after all.
“The neutrino became the prime example of hot 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,” said Keith Olive, a researcher involved in the study.
Testing the Theory
The team now plans to test their ideas through experiments on Earth, using particle accelerators and searching for evidence in the early universe. If successful, this could unlock a clearer picture of the universe’s earliest moments.
“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, another researcher on the project. The hunt for dark matter is far from over, but this new line of inquiry could finally bring us closer to solving one of cosmology’s biggest mysteries.
Ultimately, the discovery of hot dark matter would not only reveal the true nature of this elusive substance but also force cosmologists to reassess their fundamental understanding of the universe’s evolution.
