What is dark matter? It is one of the biggest mysteries in science. Astronomers know it exists because its gravity affects galaxies and the movement of stars. In fact, dark matter makes up about 85% of all the matter in the universe. But despite decades of research, scientists still don't know what it is made of.
Many researchers believe dark matter is made of unknown particles that have never been detected. However, another exciting possibility has been gaining attention—primordial black holes. These are tiny black holes that may have formed just after the Big Bang, long before the first stars and galaxies appeared.
Now, a new study by Kawasaki, Murai, and Tsuchida has provided strong support for this idea. Using powerful computer simulations, the researchers found that a special model of the early universe could naturally create enough primordial black holes to explain all of the universe's dark matter. Even more exciting, their study predicts a special type of gravitational wave that future space missions may be able to detect.
What Are Primordial Black Holes?
When most people think of black holes, they imagine giant objects formed after massive stars die. These black holes can be several times heavier than the Sun.
Primordial black holes are very different.
Scientists believe they may have formed less than a second after the Big Bang, during a time when the universe was extremely hot, dense, and expanding very rapidly.
If some regions of the young universe became much denser than the areas around them, gravity could have squeezed those regions into tiny black holes.
These black holes are called primordial black holes because they were created in the earliest moments of the universe.
For many years, scientists have wondered whether these ancient black holes could make up dark matter. If enough of them formed, they could still be floating through space today, silently affecting galaxies through their gravity.
Looking Back at the Beginning of the Universe
To understand how primordial black holes could form, scientists study a period called cosmic inflation.
Inflation happened just after the Big Bang. During this time, the universe expanded at an incredibly fast rate in a tiny fraction of a second.
This rapid expansion was not perfectly smooth. Tiny quantum fluctuations were stretched across space. Most of these small fluctuations later became the galaxies and galaxy clusters we see today.
But if some fluctuations became much larger than normal, they could collapse under their own gravity and form primordial black holes.
The challenge is finding a realistic model that can create these unusually large fluctuations.
The Bumpy Axion Inflation Model
In the new study, the researchers focused on a theory called bumpy axion inflation.
This model includes a hypothetical particle called an axion, which is believed to drive inflation in the early universe.
Instead of moving across a smooth energy landscape, the axion moves across a surface filled with plateaus and steep cliffs.
You can imagine riding a bicycle on a road. If the road is flat, your speed stays almost the same. But if you suddenly ride down a steep hill, you quickly speed up.
Something similar happens in this model.
As the axion rolls down the steep parts of the energy landscape, it suddenly speeds up. This temporary burst of speed plays a very important role.
A Burst of Particle Production
The axion is connected to another field called a U(1) gauge field.
When the axion speeds up, it rapidly produces large numbers of gauge particles.
These particles don't simply disappear. Instead, they push back on the axion itself. Scientists call this effect backreaction.
This backreaction changes how inflation continues.
Most importantly, it creates much larger density fluctuations than would normally occur.
These large fluctuations are exactly what scientists need to produce primordial black holes.
Why Powerful Computer Simulations Were Needed
Calculating this process is extremely difficult.
The interaction between the axion and the gauge particles becomes very complex, especially when backreaction becomes strong.
Simple mathematical equations are no longer accurate enough.
To solve this problem, the researchers used lattice simulations.
A lattice simulation divides space into millions of tiny sections. The computer then calculates how every field changes over time in each section.
This method allows scientists to follow the complicated behavior of the universe much more accurately.
These simulations require enormous computing power, but they provide a much clearer picture of what happened during inflation.
First, They Tested Their Method
Before studying the new inflation model, the researchers wanted to make sure their computer simulations were reliable.
They first applied their calculations to two well-known inflation models that had already been studied by other scientists.
The results matched previous research very closely.
This gave the team confidence that their simulations were working correctly.
Only after confirming this did they study the more complicated bumpy axion inflation model.
A Sharp Peak Appears
When the researchers ran the simulations, they discovered something remarkable.
As the axion moved through one of the steep cliffs in the energy landscape, particle production suddenly became extremely strong.
This caused a sharp increase in the curvature power spectrum, which measures the size of density fluctuations in the early universe.
Instead of increasing everywhere, the fluctuations became very large only over a narrow range of scales.
This is exactly the kind of pattern needed to create primordial black holes without affecting the rest of the universe.
The simulations showed that these fluctuations became strong enough for gravity to collapse them into black holes shortly after inflation ended.
Enough Black Holes to Explain Dark Matter
The researchers then calculated how many primordial black holes would form.
Their results showed that the model naturally produces black holes with masses between about one quadrillionth and one hundred trillionth of the Sun's mass.
Although these black holes are incredibly tiny compared to ordinary black holes, they could exist in enormous numbers throughout the universe.
Most importantly, the total amount of these primordial black holes is large enough to explain all of the universe's dark matter.
If this idea is correct, dark matter may not be made of mysterious new particles after all. Instead, it could simply be countless tiny black holes that have existed since the birth of the universe.
The Study Also Predicts Gravitational Waves
The research offers another exciting prediction.
The same process that creates these large density fluctuations also produces gravitational waves—tiny ripples in space and time.
The researchers found that these gravitational waves should have very specific frequencies, mainly in the microhertz (μHz) to millihertz (mHz) range.
Current ground-based detectors cannot observe these frequencies.
However, future space-based gravitational-wave observatories are expected to be sensitive enough to detect them.
If scientists observe these gravitational waves in the future, they would provide strong evidence that primordial black holes really formed during inflation.
Why This Research Is Important
This study is exciting because it connects several of the biggest mysteries in modern physics.
It provides a realistic explanation for how primordial black holes could form naturally in the early universe. It also shows that these black holes could make up all of dark matter, solving one of the greatest puzzles in astronomy.
Unlike earlier studies that relied mostly on simplified calculations, this research used detailed lattice simulations to follow the complex interactions between the inflaton field and gauge particles. This makes the results much more reliable.
Perhaps most importantly, the theory can be tested. Future space missions searching for gravitational waves may either confirm or rule out this idea.
If those predicted signals are detected, scientists could finally uncover the true nature of dark matter and learn that tiny black holes created just after the Big Bang have been quietly shaping the universe for nearly 14 billion years.
Reference: Masahiro Kawasaki, Kai Murai, Shunsuke Tsuchida, "Lattice study of primordial black hole formation in bumpy axion inflation", Arxiv, 2026. https://arxiv.org/abs/2607.01780
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