Dark matter is one of the biggest mysteries in modern science. Scientists believe it makes up about 85% of all the matter in the universe, yet no one has ever seen it directly. It does not produce, reflect, or absorb light, making it completely invisible. Even though we cannot see dark matter, scientists know it exists because of its strong gravitational effects on galaxies and other cosmic objects.
For many years, scientists have used a model called the Lambda Cold Dark Matter (ΛCDM) model to explain how the universe formed and evolved. This model has been very successful in explaining many observations. However, it still cannot explain everything. One important problem is known as the σ₈ (sigma-eight) tension, where different observations of the universe give different results for how matter is spread across space.
A new study by researchers Figueruelo, Anabella, and Pannia presents a fresh idea that could help solve this mystery. They have developed a new cosmological model called γΛCDM (Gamma Lambda Cold Dark Matter). Their research shows that a small interaction between dark matter and ordinary matter may reduce this disagreement and improve our understanding of the universe.
A New Type of Interaction
In the new γΛCDM model, dark matter and ordinary matter, also called baryons, can interact in a very special way. Ordinary matter includes everything we can see, such as stars, planets, gas, dust, and even our own bodies.
The interaction between dark matter and baryons is called pure momentum transfer. This means they exchange motion, but they do not exchange energy.
Imagine two people skating on ice. When one person gently pushes the other, both change their speed or direction. They exchange momentum, but no extra energy is created. The researchers believe something similar may happen between dark matter and ordinary matter in the universe.
This interaction is extremely weak, but over billions of years it can affect how galaxies and other large structures grow.
Only a Small Change to the Standard Model
One of the best features of the new model is that it does not completely replace the standard ΛCDM model. Instead, it makes only a small change.
The researchers found that only the Euler equations, which describe how matter moves under different forces, need to be modified. All the other important equations remain exactly the same.
This is good news because the ΛCDM model already explains many observations very well. The new model keeps all of those successful predictions while adding a small interaction that may solve existing problems.
Testing the Model with Computer Simulations
Modern cosmology depends on powerful computer programs that simulate how the universe has evolved since the Big Bang.
The research team tested their new model using three popular simulation programs:
CLASS
CAMB
SymBoltz
These programs are called Boltzmann solvers. They calculate how tiny changes in the early universe eventually became today's galaxies, stars, and galaxy clusters.
Two of these programs, CLASS and CAMB, use a mathematical shortcut called the Tight-Coupling Approximation. This shortcut helps scientists perform calculations more quickly during the early stages of the universe.
Since the new interaction affects ordinary matter, the researchers updated this approximation so that it correctly included the new physics. SymBoltz does not use this shortcut, making it a good tool to independently test the results.
After running many simulations, the researchers found that all three programs produced almost identical results.
This agreement is very important because it shows that the new model is mathematically correct and works reliably across different scientific software.
Completing a Family of Cosmological Models
The γΛCDM model is the latest member of a group of interacting cosmological models developed by the researchers.
Earlier studies introduced two similar models called αCDM and βCDM, which explored different types of interactions involving dark matter.
Now the γΛCDM model studies interactions between dark matter and ordinary matter.
Although these models are different, they all have one common effect. They slow down the growth of cosmic structures slightly.
However, the γΛCDM model works differently from the previous models.
The earlier models become important mainly during the later stages of the universe.
The γΛCDM model starts affecting the universe much earlier, even before the Cosmic Microwave Background (CMB) was formed. This means it leaves unique signals that future space missions and telescopes may be able to detect.
Reducing the Growth of Cosmic Structures
One of the most interesting results of the study is that the interaction slows the growth of matter on smaller scales.
In the early universe, ordinary matter was strongly connected with radiation. Because of this connection, ordinary matter experienced pressure.
When dark matter interacts with baryons through momentum transfer, it experiences a tiny drag force. This drag slightly slows down the growth of dark matter structures.
As a result, galaxies and galaxy clusters grow a little more slowly than predicted by the standard ΛCDM model.
This reduced growth matches some astronomical observations much better.
Most importantly, it helps reduce the σ₈ tension, one of the biggest unsolved problems in cosmology today.
Comparing the Model with Real Observations
A scientific model is only useful if it agrees with real observations.
To test their idea, the researchers compared their predictions with several major astronomical datasets.
These included observations from:
The Cosmic Microwave Background (CMB)
Baryon Acoustic Oscillations (BAO)
Type Ia Supernovae
Sunyaev-Zeldovich (SZ) galaxy cluster counts
Dark Energy Survey (DES) Year 6 data
Using these observations, they measured how strong the interaction between dark matter and baryons could be.
When only the standard datasets were used, the interaction had to be very small.
However, when additional observations from nearby galaxies and galaxy clusters were included, something interesting happened.
The new γΛCDM model performed slightly better than the standard ΛCDM model.
The researchers even found evidence for a small but non-zero interaction between dark matter and baryons. Although the evidence is not yet strong enough to claim a discovery, it is an exciting result that deserves further investigation.
Studying the Effect of Neutrinos
The researchers also wanted to know whether another particle, called the neutrino, could produce similar effects.
Neutrinos are tiny particles with very small masses. They also slow the growth of cosmic structures.
The team compared the effects of neutrinos with the new dark matter interaction.
They found that, in most cases, scientists can clearly tell the difference between the two effects.
Only when certain observations, especially CMB lensing data, are removed do the two effects become slightly similar.
This means future observations will help scientists determine whether the reduced growth of cosmic structures is caused by neutrinos, dark matter interactions, or both.
Why This Research Is Important
The γΛCDM model is important because it offers a simple extension to the standard cosmological model without changing the overall history of the universe.
Instead of changing how the universe expands, it changes only how matter moves and forms structures.
This makes it a very attractive idea because it keeps the successful predictions of ΛCDM while providing a possible explanation for one of cosmology's biggest puzzles.
The study also shows that the model works consistently in different scientific computer programs, increasing confidence in its predictions.
Looking to the Future
The researchers believe that future space missions and large sky surveys will provide much more accurate measurements of the universe.
These observations will allow scientists to test the γΛCDM model more carefully.
If future data continue to support the idea of momentum transfer between dark matter and ordinary matter, it could become one of the most important discoveries in modern cosmology.
Although more evidence is needed, this research opens an exciting new direction for understanding the universe. It suggests that dark matter may not be completely isolated after all. Even a tiny interaction with ordinary matter could have shaped the formation of galaxies and helped create the universe we see today.
As scientists continue exploring the mysteries of dark matter, models like γΛCDM provide valuable new ideas that could eventually reveal the true nature of the invisible matter that dominates our universe.
Reference: David Figueruelo, Florencia Anabella Teppa Pannia, "Dark matter-baryons elastic coupling", Arxiv, 2026. https://arxiv.org/abs/2607.22415
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