Dark matter is one of the biggest mysteries in modern science. Scientists believe that it makes up about 85% of all matter in the Universe, yet it cannot be seen directly. It does not emit light, reflect light, or absorb radiation. The only way we know it exists is through its gravitational effects on galaxies and cosmic structures.
For many years, scientists assumed that dark matter behaves like a collisionless substance. This means dark matter particles move through space without interacting much with other particles. They mainly influence the Universe through gravity.
However, a new study by Dallari and colleagues suggests something surprising. Their research proposes that dark matter might start interacting with another invisible component called dark radiation later in the history of the Universe. This new idea is called dark matter recoupling, and it could change how scientists think about the evolution of the cosmos.
The Traditional Picture of Dark Matter
In the standard model of cosmology, dark matter plays a very important role in forming structures in the Universe.
After the Big Bang, small fluctuations in matter slowly grew under gravity. Over billions of years, these fluctuations formed:
galaxies
galaxy clusters
the large cosmic web structure of the Universe
Dark matter is believed to act like the invisible scaffolding that holds these structures together.
To explain observations of the Universe, scientists usually assume that dark matter particles do not collide or interact strongly with other particles. If they did interact frequently, they would transfer momentum to other particles, which would slow down the growth of cosmic structures.
This would leave clear signals in astronomical observations.
Because these signals are not seen, dark matter is usually considered almost perfectly collisionless.
Testing Dark Matter with Cosmic Observations
Scientists test the behavior of dark matter using several powerful observations.
One of the most important is the Cosmic Microwave Background (CMB). This is the faint radiation left over from the early Universe, about 380,000 years after the Big Bang. It acts like a snapshot of the young Universe.
If dark matter interacted strongly with other particles during that time, the pattern of the CMB would look different. Because the observed pattern matches the collisionless model very well, scientists concluded that dark matter interactions must have been extremely weak in the early Universe.
Other observations also help test dark matter behavior, including:
the Lyman-alpha forest, which studies light from distant quasars
the distribution of small satellite galaxies around the Milky Way
the large-scale distribution of galaxies
Together, these measurements provide strong limits on how much dark matter can interact.
The Idea of Dark Radiation
The new research focuses on a possible interaction between dark matter and a hypothetical particle called dark radiation.
Dark radiation is similar to ordinary radiation, like photons or neutrinos, because it moves at nearly the speed of light. However, it would belong to the dark sector, meaning it does not interact with normal matter.
If dark matter interacts with dark radiation, the two types of particles could scatter off each other and exchange momentum.
This interaction would influence how matter clumps together over time.
In many previous models, such interactions were strongest in the early Universe and then became weaker as the Universe expanded.
This process is called decoupling.
A Completely Different Possibility
Dallari and the research team explored a very different idea.
Instead of dark matter interactions becoming weaker over time, they asked: What if the interaction becomes stronger later in the Universe?
In their model, dark matter and dark radiation interact very weakly in the early Universe, which keeps the Cosmic Microwave Background unchanged.
But as the Universe grows older, the interaction slowly becomes stronger.
Eventually, dark matter could start interacting with dark radiation billions of years after the Big Bang.
This process is called dark matter recoupling, because the particles begin interacting again after being mostly separate for a long time.
Why This Matters
When dark matter particles interact with dark radiation, they exchange momentum. This affects how dark matter moves and clusters in space.
If the interaction becomes significant, it can slow down the growth of cosmic structures. This effect would appear in measurements of how galaxies and matter are distributed across the Universe.
One important prediction of the recoupling model is a scale-dependent suppression of structure growth. In simple terms, some sizes of cosmic structures would grow more slowly than others.
Interestingly, this effect would appear late in the history of the Universe, usually after a period called reionization, when the first stars and galaxies began lighting up the cosmos.
Because the interaction happens so late, it would not strongly affect the Cosmic Microwave Background. Instead, scientists must look at large-scale structures to detect it.
Observations Used in the Study
To test their idea, the researchers compared the recoupling model with real observational data.
They used information from:
Cosmic Microwave Background measurements
Baryon Acoustic Oscillations (BAO), which track patterns in the distribution of galaxies
These observations help scientists understand how matter has evolved across cosmic time.
By analyzing this data, the researchers were able to place limits on how strong the dark matter–dark radiation interaction could be today.
What the Results Show
The results revealed something interesting.
If all dark matter particles interact with dark radiation, then the interaction must still be very weak today. Otherwise, the structure of the Universe would not match what astronomers observe.
This means that even in the recoupling scenario, most dark matter still behaves almost collisionlessly.
However, the data also showed that there is still some room for a different possibility.
Scientists cannot rule out the idea that a small fraction of dark matter behaves differently.
The study suggests that up to about 4% of dark matter could still be strongly interacting with dark radiation today.
This small interacting component would not dramatically change the overall structure of the Universe, but it could produce subtle effects that future observations might detect.
A Simple Particle Physics Model
The researchers also explored how this recoupling behavior could arise from a realistic particle physics model.
In their example:
dark matter particles are fermions
dark radiation particles are light scalar particles
the interaction between them occurs through a Yukawa-type force
This type of interaction is common in particle physics.
Interestingly, the same interaction that causes recoupling might also help explain how dark matter formed in the early Universe through a process called thermal freeze-out.
This creates a link between early-Universe physics and late-time cosmology, making the model especially interesting.
However, the study found that the mass of the dark radiation particle may require fine tuning to fit current observations.
Future Experiments Could Test the Idea
The next generation of galaxy surveys could help scientists investigate dark matter recoupling more closely.
Upcoming projects include:
DESI (Dark Energy Spectroscopic Instrument)
Euclid space mission
Vera Rubin Observatory
These powerful instruments will measure the positions of millions of galaxies and map the structure of the Universe with incredible precision.
Such data will allow scientists to detect small changes in how cosmic structures grow, which could reveal whether dark matter recoupling is happening.
A New Perspective on the Dark Universe
Dark matter remains one of the greatest puzzles in science. Even though it dominates the matter content of the Universe, its true nature is still unknown.
The idea of dark matter recoupling suggests that the dark sector might be more complex than previously thought.
Instead of being completely isolated forever, dark matter might start interacting again at late times, shaping the structure of the Universe in subtle ways.
While most dark matter likely remains collisionless, the possibility that a small interacting fraction exists opens a new path for research.
As new astronomical data arrives in the coming years, scientists may finally learn whether dark matter truly stays silent — or whether it begins to communicate with other hidden particles as the Universe grows older.
Reference: Eugenia Dallari, Francesco Castagna, Emanuele Castorina, Maria Archidiacono, Ennio Salvioni, "Dark Matter Recoupling", Arxiv, 2026. https://arxiv.org/abs/2603.09969
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