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For about 50 years, the scientific community has been struggling with a fundamental problem: there is not enough visible matter in the universe.

All the matter we can see — stars, planets, space dust and everything in between — can’t explain why the universe behaves the way it does, and there needs to be five times as much of it around for the researchers’ observations to make sense, according to NASA . Scientists call this dark matter because it does not interact with light and is invisible.

In the 1970s, American astronomers Vera Rubin and W. Kent Ford confirmed the existence of dark matter by observing stars orbiting the edge of spiral galaxies. They noted that these stars were moving too fast to be held together by the visible matter of the galaxy and its gravity—instead, they had to fly sideways. The only explanation was a large amount of invisible matter binding the galaxy.

“What you see in a spiral galaxy,” Rubin said at the time, “is not what you get.” Her work was based on a hypothesis formulated in the 1930s by Swiss astronomer Fritz Zwicky and gave impetus to the search for the elusive substance.

Since then, scientists have been trying to observe dark matter directly and have even built large devices to detect it – but so far without success.

At the beginning of the search, the famous British physicist Stephen Hawking postulated that dark matter could be hidden in black holes – the main topic of his work – formed during the big bang.

Now, a new study by MIT researchers has brought the theory back into the spotlight, revealing what these primordial black holes are made of and potentially discovering a whole new type of exotic black hole in the process.

“It was a really great surprise in that way,” said David Kaiser, one of the study’s authors.

“We used Stephen Hawking’s famous calculations about black holes, especially his important result about the radiation that black holes emit,” Kaiser said. “These exotic black holes arise from trying to deal with the dark matter problem – they are a byproduct of the dark matter explanation.”

Scientists have made many guesses about what dark matter could be, ranging from unknown particles to extra dimensions. But Hawking’s theory of black holes only recently came into play.

“People didn’t really take it seriously until maybe 10 years ago,” said study co-author Elba Alonso-Monsalve, a graduate student at MIT. “And that’s because black holes once seemed really elusive—in the early 20th century, people thought they were just a mathematical fun fact, nothing physical.”

We now know that almost every galaxy has a black hole at its center, and researchers’ discovery of Einsteinian gravitational waves created by colliding black holes in 2015 – a remarkable discovery – made it clear that they are everywhere.

“In fact, the universe is teeming with black holes,” Alonso-Monsalve said. “But the dark matter particle has not been found, even though people have looked in all the places they expected to find it. This does not mean that dark matter is not a particle or that it is certainly a black hole. It could be a combination of both. But black holes as candidates for dark matter are now being taken much more seriously.

Other recent studies have confirmed the validity of Hawking’s hypothesis, but work by Alonso-Monsalve and Kaiser, MIT’s Germeshausen Professor of Physics and History of Science, goes a step further and examines exactly what happened when the primordial black first holes formed.

The study, published June 6 in the journal Physical Review Letters, reveals that these black holes must have appeared in the first quintillionth of a second after the big bang: “This is really early and much earlier than when protons and neutrons , the particles from which everything is made have formed,” Alonso-Monsalve said.

In our everyday world, we can’t find protons and neutrons broken up, she added, and they act like elementary particles. However, we know they are not because they are made up of even smaller particles called quarks held together by other particles called gluons.

“Right now you can’t find quarks and gluons alone and free in the universe because it’s too cold,” added Alonso-Monsalve. “But at the beginning of the big bang, when it was very hot, they could be found alone and free. So the primordial black holes were formed by absorbing free quarks and gluons.

Such a formation would make them fundamentally different from the astrophysical black holes that scientists typically observe in the universe, which are the result of collapsing stars. Also, the primordial black hole would be much smaller – only the mass of an asteroid, on average, condensed into the volume of one atom. But if a sufficient number of these primordial black holes did not evaporate in the early big bang and have survived to the present day, they could represent all or most of the dark matter.

During the creation of the primordial black holes, another type of black hole unseen before must have formed as a kind of byproduct, according to the study. They would be even smaller – exactly the mass of a rhinoceros condensed into a volume smaller than the volume of a single proton.

These miniature black holes, because of their tiny size, could pick up a rare and exotic property from the quark-gluon soup in which they formed, called “color charge.” It’s a charge state that’s exclusive to quarks and gluons, never found in ordinary objects, Kaiser said.

This color charge would make them unique among black holes, which normally have no charge at all. “It is inevitable that these even smaller black holes also formed as a by-product (of the formation of the primordial black holes),” Alonso-Monsalve said, “but they would no longer be around today as they would have already evaporated .”

However, if they were still only about ten millionths of a second after the Big Bang when protons and neutrons formed, they could leave visible traces by changing the balance between the two types of particles.

“The balance of how many protons and how many neutrons are made is very delicate and depends on what other things existed in the universe at the time. If these color-charged black holes were still around, they could shift the balance between protons and neutrons (in favor of one or the other), just enough that in the next few years we’ll be able to measure that,” she added.

The measurement could come from Earth-based telescopes or sensitive instruments on orbiting satellites, Kaiser said. But there may be another way to confirm the existence of these exotic black holes, he added.

“The creation of a population of black holes is a very turbulent process that would send huge ripples into the surrounding space-time. They will be weakened in space history, but not to zero,” Kaiser said. “The next generation of gravity detectors could glimpse low-mass black holes—an exotic state of matter that is an unexpected byproduct of more common black holes, which could explain dark matter today.”

What does this mean for ongoing experiments trying to detect dark matter, such as the LZ Dark Matter Experiment in South Dakota?

“The idea that there are exotic new particles remains an interesting hypothesis,” Kaiser said. “There are other kinds of large experiments, some of which are under construction, looking for fancy ways to detect gravitational waves. And they can indeed pick up some of the stray signals from the very turbulent formation process of the primordial black holes.

There is also the possibility that primordial black holes are just part of dark matter, Alonso-Monsalve added. “It doesn’t actually have to be all the same,” she said. “There is five times as much dark matter as ordinary matter, and ordinary matter is made up of many different particles. So why does dark matter have to be a kind of object?”

Primordial black holes have regained popularity with the discovery of gravitational waves, but not much is still known about their formation, according to Nico Cappelluti, an assistant professor in the Department of Physics at the University of Miami. He did not participate in the study.

“This work is an interesting, viable option to explain the elusive dark matter,” Cappelluti said.

The research is exciting and suggests a new mechanism for the formation of the first generation of black holes, said Priyamvada Natarajan, the Joseph S. and Sophia S. Fruton Professor of Astronomy and Physics at Yale University. She also did not participate in the study.

“All the hydrogen and helium we have in our universe today was created in the first three minutes, and if enough of these primordial black holes were around by then, they would have affected this process and those effects might be noticeable,” Natarajan said.

“The fact that this is an observable hypothesis is what I find really exciting, in addition to the fact that it suggests that nature probably creates black holes starting from the earliest times in multiple ways.”