Did the Milky Way Destroy Ancient Black Hole Clusters? New Study Brings Scientists Closer to Solving the Dark Matter Mystery

Dark matter is one of the biggest mysteries in the Universe. Scientists know it exists because its gravity affects stars, galaxies, and even the large-scale structure of the cosmos. However, no one has directly detected it yet or knows exactly what it is made of.

One interesting idea is that dark matter may be made of Primordial Black Holes (PBHs). These are not ordinary black holes formed from dying stars. Instead, they may have formed just a fraction of a second after the Big Bang, when the Universe was extremely hot and dense.

For many years, scientists believed that these black holes, if they exist, would be spread throughout space as individual objects. But some theories suggest something different—they may have formed in large clusters. A new study by researchers Tkachev and Pilipenko investigates what happened to these clusters over billions of years. Their results show that many of them may have slowly broken apart due to repeated gravitational encounters inside the Milky Way.

This discovery could change the way scientists search for dark matter.

What Are Primordial Black Holes?

Most black holes are created when massive stars collapse after running out of fuel. Primordial black holes are very different. They could have formed in the first moments after the Big Bang, long before the first stars and galaxies appeared.

If these ancient black holes exist, they could make up some or even all of the Universe's dark matter.

Scientists have been searching for them using several methods, including gravitational microlensing, gravitational waves, and observations of stars. So far, no clear evidence has been found, but the search continues.

Why Do Scientists Think They Formed Clusters?

Some theories suggest that primordial black holes were not created one by one. Instead, many of them formed close together because of tiny density differences in the early Universe.

As gravity pulled them toward each other, they formed dense groups called clusters. These clusters could contain thousands or even millions of primordial black holes bound together by gravity.

If this happened, scientists would have to rethink many previous dark matter studies because a cluster behaves differently from a single black hole.

The Big Question

The researchers wanted to answer one simple question:

Can primordial black hole clusters survive for billions of years inside the Milky Way?

As these clusters travel around the galaxy, they sometimes pass close to other clusters. During these close encounters, gravity pulls on the black holes inside each cluster.

Some black holes gain enough energy to escape completely.

If this keeps happening again and again over billions of years, the clusters slowly lose members and become smaller. Eventually, much of the original cluster may disappear.

How Did Scientists Study This?

The research team combined mathematical calculations with powerful computer simulations.

First, they created a mathematical model to estimate how often clusters collide inside a Milky Way-like galaxy.

Next, they performed 72 detailed N-body simulations. These simulations calculated the motion of every object during collisions between two black hole clusters. This helped the researchers measure how much mass escaped during each encounter.

Finally, they carried out large cosmological simulations that followed the formation of a Milky Way-like galaxy from the early Universe until today.

These simulations tracked the entire collision history of every black hole cluster over nearly 13 billion years.

Early Universe Was More Dangerous

One of the biggest discoveries of the study is that most of the damage happened very early in the Universe.

Previous studies mostly looked at collisions happening today inside the Milky Way.

However, the new simulations showed that billions of years ago, the galaxy was very different. The young Milky Way was made of many small, dense regions where clusters moved more slowly.

These slower collisions were actually much better at breaking apart black hole clusters.

The researchers found that about half of the total mass lost by the clusters happened before the Universe reached about one-third of its current age.

This early destruction had been completely missed by earlier calculations.

Small Collisions Can Cause Big Changes

The simulations also showed something surprising.

Modern collisions between clusters are usually too fast to cause major damage.

The most destructive collisions happen when clusters meet at speeds between 12 and 20 kilometers per second.

Today, most encounters inside the Milky Way happen much faster than this.

As a result, each individual collision removes only a tiny amount of mass.

But over billions of years, thousands of these weak encounters slowly strip black holes away from the clusters.

Instead of one giant collision destroying a cluster, many small collisions gradually wear it down.

How Much of Each Cluster Survives?

The researchers studied two different cluster sizes.

The first type had a mass equal to about one million Suns.

The second type was much larger, with a mass of about ten million Suns.

Near our Solar System, the results were dramatic.

For the smaller clusters, only about 50% of the original mass remained inside the cluster today.

The other half had escaped into space.

The larger clusters suffered even more.

Only about 4% of their original mass remained inside the cluster.

Almost all of their black holes had been scattered into the Milky Way over billions of years.

A Smooth Population of Black Holes

As clusters lose black holes, those escaped objects do not disappear.

Instead, they spread throughout the galaxy, creating a much smoother distribution of primordial black holes.

This means the Milky Way may contain both:

• Black holes still trapped inside clusters.

• Black holes moving freely through the dark matter halo.

Scientists call this the smooth component of primordial black hole dark matter.

Understanding how much dark matter belongs to each group is extremely important.

Why Does This Matter for Dark Matter Searches?

Many dark matter experiments look for gravitational microlensing.

This happens when a black hole passes in front of a distant star. The black hole's gravity bends the star's light, making it appear brighter for a short time.

If black holes exist as single objects, scientists expect one type of microlensing signal.

But if they exist inside clusters, the signal becomes very different.

This means current experiments may not correctly identify clustered black holes.

The new study shows that the answer is even more complicated.

Some primordial black holes remain inside clusters, while many others have escaped.

Both groups must now be included when scientists analyze microlensing observations.

Looking Toward Nearby Galaxies

Many microlensing experiments observe stars in the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), two nearby galaxies.

The researchers calculated how much primordial black hole dark matter along these directions exists as isolated objects.

For smaller clusters, nearly half of the dark matter becomes part of the smooth population.

For larger clusters, this increases to about 92%.

This means most primordial black holes along these lines of sight may actually behave like isolated black holes instead of large clusters.

What Happens Next?

The researchers plan to improve their simulations even further.

Future studies will examine clusters with different sizes, different shapes, and collisions between unequal clusters.

They also want to include other physical effects, such as interactions with ordinary dark matter and additional gravitational forces.

Finally, they hope to use these improved models to reanalyze data from major microlensing surveys like EROS, OGLE, and Subaru Hyper Suprime-Cam.

These new calculations could provide much more accurate limits on how much dark matter is made of primordial black holes.

A Step Closer to Solving the Mystery

Dark matter has remained one of science's greatest unsolved mysteries for decades. Primordial black holes are still one of the most exciting possible explanations.

This new research suggests that if primordial black hole clusters formed in the early Universe, they did not remain unchanged. Over billions of years, repeated gravitational encounters inside the Milky Way slowly tore many of them apart, creating a large population of isolated black holes.

The study shows that dark matter made of primordial black holes may exist in both clustered and smooth forms. Understanding this balance is essential for interpreting future observations and improving the search for one of the Universe's most mysterious invisible ingredients.

Although the mystery of dark matter is still unsolved, studies like this bring scientists one step closer to understanding what fills most of the Universe.

Reference: M.V. Tkachev, S.V. Pilipenko, "Do Primordial Black Hole Clusters Survive the Galaxy? Collisional Disruption and Microlensing Implications", Arxiv, 2026. https://arxiv.org/abs/2607.09918

Technical Terms 

1. Dark Matter

Dark matter is an invisible type of matter that does not produce, reflect, or absorb light. We cannot see it directly, but scientists know it exists because its gravity affects stars and galaxies. It makes up about 85% of all matter in the Universe.

2. Primordial Black Holes (PBHs)

Primordial black holes are ancient black holes that may have formed just after the Big Bang, long before stars and galaxies existed. Unlike normal black holes, they were not created by collapsing stars.

3. Black Hole Cluster

A black hole cluster is a group of many black holes held together by gravity, similar to how stars are held together in a star cluster.

4. Milky Way Halo

The Milky Way halo is a huge, invisible region surrounding our galaxy. It contains most of the galaxy's dark matter and extends far beyond the visible stars.

5. Gravitational Encounter

A gravitational encounter happens when two objects pass close to each other. Their gravity pulls on one another, changing their motion. Even without crashing, they can exchange energy.

6. Cluster Disruption

Cluster disruption means a black hole cluster slowly breaks apart. During repeated gravitational encounters, some black holes gain enough energy to escape from the cluster.

7. Diffuse or Smooth Component

When black holes escape from a cluster, they spread throughout the galaxy instead of staying together. This scattered population is called the smooth or diffuse component.

8. Microlensing

Gravitational microlensing occurs when a black hole passes in front of a distant star. Its gravity bends the star's light, making the star appear brighter for a short time. Scientists use this effect to search for invisible objects like black holes.

9. Gravitational Lens

A gravitational lens is any massive object whose gravity bends light, just like a glass lens bends light. Black holes can act as powerful gravitational lenses.

10. N-body Simulation

An N-body simulation is a computer model that calculates how many objects move under gravity. It helps scientists study how stars, galaxies, or black hole clusters evolve over billions of years.

11. Cosmological Simulation

A cosmological simulation is a large computer simulation that recreates the evolution of the Universe from shortly after the Big Bang to the present day.

12. Redshift (z)

Redshift tells scientists how far back in time they are looking.

• z = 0 means the present-day Universe.

• Higher redshift (like z = 9) means the Universe was much younger.

13. NFW (Navarro-Frenk-White) Profile

The NFW profile is a mathematical model that describes how dark matter is distributed inside galaxies. It predicts that dark matter is denser near the center and becomes less dense farther away.

14. Solar Circle

The Solar Circle is the region around the Milky Way where our Sun is located, about 26,000 light-years from the Galactic Center.

15. Large Magellanic Cloud (LMC)

The Large Magellanic Cloud is a nearby dwarf galaxy that orbits the Milky Way. Scientists often observe its stars in microlensing experiments.

16. Small Magellanic Cloud (SMC)

The Small Magellanic Cloud is another nearby dwarf galaxy and is also used in dark matter and microlensing studies.

17. Gravitational Waves

Gravitational waves are tiny ripples in space-time created when massive objects like black holes collide. They were first detected in 2015.

18. Hubble Time

Hubble time is approximately the age of the Universe, about 13.8 billion years.

19. Collision Rate

The collision rate tells scientists how often black hole clusters pass close enough to each other to interact through gravity.

20. Mass Fraction

Mass fraction simply means what percentage of the total mass remains or belongs to a particular group. For example, if a cluster has a surviving mass fraction of 50%, it has lost half of its original mass.

21. Hierarchical Galaxy Formation

This is the idea that small galaxies formed first after the Big Bang and later merged together over billions of years to create larger galaxies like the Milky Way.

22. Dark Matter Halo

A dark matter halo is a giant, invisible cloud of dark matter surrounding a galaxy. Its gravity helps hold the galaxy together and influences the motion of stars.

23. Einstein Radius

The Einstein radius is the region around a massive object where its gravity bends light strongly enough to produce gravitational lensing. A larger object, such as a black hole cluster, has a larger Einstein radius than a single black hole.

24. Escape Fraction

Escape fraction is the percentage of black holes that gain enough energy during an encounter to leave their cluster forever.

25. Velocity Scale

Velocity scale is the typical speed at which objects move inside a black hole cluster. Scientists compare collision speeds with this value to determine how disruptive an encounter will be.