Dark matter constitutes one of the great mysteries of modern cosmology. Invisible yet omnipresent, it shapes galaxy formation and the structure of the Universe through its gravity alone. While it is accepted that it would be a particle, like the quark or electron, decades of experimental research have so far yielded no direct observation.
A new theoretical clue, developed by Yann Mambrini, a researcher at IJCLab, and his collaborators, now proposes a profound re-examination of dark matter's birth scenario, placing it at the heart of the very first fractions of a second of the Universe.
The collaboration between IJCLab and the University of Minnesota presented its "UFO" theory, for
Ultra-relativistic Freeze-Out, in December 2025 in the journal
Physical Review Letters. To understand it, we must return to the events that structured the Primordial Universe, namely the mysterious first instants of the Universe.
According to the most compelling theories, the Universe underwent a spectacular phase of inflation between 10
-36 and 10
-33 seconds after the Big Bang, during which the Universe would have swollen by a factor of at least 10
26. Inflation would have resulted in the formation of a primordial plasma, a thermal bath from which most of the particles in the Universe would have emerged.
It is in this initial plasma that scientists generally locate the production of dark matter, assumed to be "cold" and generated in thermal equilibrium with ordinary matter. The energy of the plasma would produce dark matter in equal quantities with this observable matter, until dark matter production decoupled when the plasma energy was no longer sufficient to produce both.
This hypothesis is attractive for theorists because it predicts dark matter that is both cold enough (and thus static) to provide a framework for large cosmic structures, while being less dense than ordinary matter.
The famous WIMPs (Weakly Interacting Massive Particles), massive particles that would interact with ordinary matter via the weak interaction, rely on this scheme. But, after decades of searching, the persistent absence of a signal in direct detection experiments tends to weaken this hypothesis. "
Over the decades, experiments have extended the exclusion zones to the point of making the existence of WIMPs highly improbable," explains Yann Mambrini. "
This tension motivates us to change our perspective on dark matter production mechanisms and in particular their timing."
This is precisely what the UFO theory developed by this team proposes. It suggests that dark matter could have been generated very early, even before the initial plasma formed, from the decay of inflatons, hypothetical particles that would be responsible for the violent inflation of the Universe. This matter would initially be "ultra-relativistic," meaning very hot and extremely fast, but its energy would have been immediately diluted by the violent expansion of the primordial Universe.
As a result, it would have cooled almost instantaneously, becoming compatible with the formation of large cosmic structures. "
The matter we propose is hotter, but it dilutes so quickly that it loses its energy much earlier than imagined, traveling very little distance, which allows galaxies to form around it," details Yann Mambrini.
The formation mechanisms of this "UFO" dark matter constrain the physical properties of the particles that could correspond to it. Lighter than traditional WIMPs, with masses on the order of MeV, the particles proposed by this theory would retain a non-negligible coupling with ordinary matter, through the weak interaction, unlike so-called "FIMP" scenarios (Feebly Interacting Massive Particles), which have been in vogue for several years but whose extraordinarily weak interactions did not allow for observations.
Happy coincidence: the mass and interaction range of these particles would allow their detection by certain dark matter detection devices currently operating or in the project stage. "
After spending several years studying the link between primordial plasma formation and dark matter, the fact that our result is both coherent and experimentally testable is extremely stimulating," says Yann Mambrini.
Among the candidate experiments are XENON, which has been operating for several years, DAMIC-M, a prototype of which is currently in action, or TESSERACT, which will be installed in a few years at LSM. An adjustment of the settings of these experiments would be enough to hunt for these promising "UFOs."
In the longer term, direct production of UFO-type particles could even be considered in particle accelerators, via the search for missing energy signatures. Experimental work alone is capable of validating this theory, or disproving it, like many dark matter models before it.