Scientists have long believed that the universe follows a model called Lambda Cold Dark Matter (ΛCDM). This model explains how galaxies, stars, and large cosmic structures formed after the Big Bang. For many years, it has matched observations very well.
However, recent studies have found something interesting. The universe may not be growing exactly as the standard model predicts. Large structures such as galaxies and galaxy clusters appear to be forming a little more slowly than expected. While the difference is still small, it has encouraged scientists to look for new ideas that could explain what is happening.
A new study by physicists Matthew Schmaltz and Karthik Sivarajan suggests that dark matter might be interacting with a hidden form of radiation. This interaction could act like a drag force, slowing down the growth of structures in the universe.
The Mystery of Cosmic Growth
The universe is filled with galaxies that are grouped together into enormous cosmic structures. Over billions of years, gravity pulls matter together, allowing these structures to grow.
Astronomers measure how fast this growth happens using several techniques. Two important methods are called fσ8 measurements and weak gravitational lensing.
Recent observations suggest that the growth of structures in the nearby universe may be slightly slower than what the ΛCDM model predicts.
This doesn't mean the standard model is wrong. But it may mean that something extra is happening that scientists have not yet included in their calculations.
What Is Dark Matter?
Dark matter is one of the biggest mysteries in science.
It does not produce light, reflect light, or absorb light. Because of this, scientists cannot see it directly. However, they know it exists because its gravity affects stars, galaxies, and galaxy clusters.
In fact, dark matter makes up about 85% of all matter in the universe.
Without dark matter, galaxies would not have formed the way they did.
Even though dark matter is everywhere, scientists still do not know exactly what it is made of.
Can Dark Matter Decay?
One idea is that dark matter slowly decays over time.
If dark matter disappears little by little, there would be less matter available to build galaxies and galaxy clusters. This sounds like a possible explanation for the slower growth seen in observations.
But there is a problem.
Imagine two regions of space: one with lots of matter and one with less matter. If dark matter decays at the same rate everywhere, both regions lose the same percentage of matter.
As a result, the difference between dense and less dense regions remains almost unchanged.
This means dark matter decay alone cannot significantly slow the growth of cosmic structures.
Introducing Dark Radiation
The researchers explored another possibility called dark radiation.
Dark radiation is a hypothetical form of radiation that does not interact with ordinary light. It would be invisible to telescopes.
In the new model, dark matter slowly decays and produces dark radiation.
The important part is that dark matter can also interact with this dark radiation.
When these interactions happen, dark matter experiences a drag force, similar to how air resistance slows down a moving object.
This drag makes it harder for dark matter to fall into gravitational wells and form larger structures.
As a result, the growth of galaxies and galaxy clusters slows down.
Why Previous Models Didn't Work
Scientists have studied dark matter and dark radiation interactions before.
In most earlier models, dark radiation was created in the early universe shortly after the Big Bang.
As the universe expanded, this radiation became weaker and weaker.
Because of this, the drag force was strongest in the distant past and almost disappeared today.
However, current observations suggest that any slowing of structure growth is happening mainly in the recent universe.
The old models therefore do not match the observations very well.
A New Solution
Schmaltz and Sivarajan proposed a different idea.
Instead of producing all the dark radiation early in cosmic history, their model creates dark radiation continuously.
Dark matter slowly decays over billions of years, constantly adding new dark radiation to the universe.
This means the amount of dark radiation does not fade away as quickly as in previous models.
As fresh dark radiation keeps appearing, the drag force acting on dark matter remains important even at late times.
In fact, the drag effect can become stronger relative to the expansion of the universe.
This is exactly what scientists need to explain the slower growth seen in recent observations.
The iDCDM Model
The researchers call their theory Interacting Decaying Cold Dark Matter, or iDCDM.
The model introduces only two new quantities:
The rate at which dark matter decays.
The strength of the interaction between dark matter and dark radiation.
Despite adding these new features, the model leaves many successful predictions of standard cosmology unchanged.
It does not significantly affect:
The expansion history of the universe.
The formation of light elements after the Big Bang.
The Cosmic Microwave Background, the ancient light left over from the early universe.
Instead, the model mainly affects how structures grow over time.
A Unique Prediction
One of the most exciting things about the iDCDM model is that it makes a clear prediction.
The slowing of structure growth should not happen equally on all scales.
Large cosmic structures should remain almost unaffected.
Smaller structures should experience a noticeable slowdown.
This creates what scientists call a step-shaped suppression in the growth rate.
Think of it like a staircase.
On one side, growth remains normal. On the other side, growth becomes weaker.
This pattern would be a unique signature of dark matter interacting with dark radiation.
Could Neutrinos Be Responsible?
Scientists know that neutrinos can also slow the growth of structures.
Neutrinos are tiny particles that move through space at extremely high speeds.
Because they move so quickly, they can reduce the formation of small-scale structures.
However, neutrinos affect the universe differently from the new dark matter model.
Their influence started much earlier in cosmic history and follows a different pattern.
Future observations should be able to tell the difference between neutrino effects and the drag force predicted by iDCDM.
What Do Current Observations Say?
The researchers tested their model using data from several major cosmological surveys.
They compared the predictions with observations of galaxy clustering, gravitational lensing, supernovae, and the Cosmic Microwave Background.
The results showed a small preference for the new model compared with standard ΛCDM.
The improvement is not large enough to prove the theory is correct.
However, it suggests that the idea deserves further investigation.
At the moment, scientists simply do not have enough data to make a final decision.
Future Surveys Will Decide
The next few years could provide the answer.
New observatories and surveys such as the Dark Energy Spectroscopic Instrument (DESI), Euclid, and the Vera C. Rubin Observatory will measure the growth of cosmic structures with unprecedented precision.
These projects will map millions of galaxies across the universe and reveal how structures evolved over time.
If they detect the predicted step-shaped slowdown, it would provide strong evidence for interactions between dark matter and dark radiation.
If they do not find the effect, the iDCDM model may be ruled out.
Looking Ahead
Dark matter remains one of the greatest mysteries in science. Although it shapes the universe through gravity, its true nature is still unknown.
The iDCDM model offers a fascinating possibility. It suggests that dark matter may not be completely silent and isolated. Instead, it could be interacting with a hidden world of dark radiation created by its own slow decay.
Upcoming observations will test this idea more accurately than ever before. If the prediction is confirmed, it would reveal entirely new physics and give scientists a deeper understanding of the invisible universe that surrounds us.
Reference: Martin Schmaltz, Eashwar N. Sivarajan, "Dark Matter with a Drag at Low Redshift", Arxiv, 2026. https://arxiv.org/abs/2606.12521
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