For half a century, the scientific realm has been wrestling with a perplexing enigma: the universe appears to be lacking in visible matter. The entirety of observable matter—comprising stars, planets, interstellar dust, and everything else—falls short of explaining the universe's behavior. According to NASA, there should be a fivefold increase in this matter for the researchers' observations to align with theoretical predictions.

This unseen mass is termed "dark matter," as it neither engages with light nor is it perceptible. In the 1970s, American astronomers Vera Rubin and W. Kent Ford substantiated the existence of dark matter by scrutinizing the motion of stars at the periphery of spiral galaxies. They observed that these stars were orbiting at velocities too high to be maintained by the galaxy's visible matter and its gravitational pull; they should have been dispersing instead.

The sole plausible explanation was the presence of a substantial amount of unseen matter, holding the galaxy together. Rubin remarked at the time, "What you see in a spiral galaxy is not what you get." Her research expanded on a hypothesis proposed in the 1930s by Swiss astronomer Fritz Zwicky and ignited the quest for this elusive substance. Since then, scientists have endeavored to directly observe dark matter and have even constructed sizable instruments to detect it—yet, to no avail.

At the onset of this search, the renowned British physicist Stephen Hawking theorized that dark matter might be concealed within black holes—the central focus of his research—formed during the Big Bang. Now, a novel study by researchers at the Massachusetts Institute of Technology has reignited interest in this theory, shedding light on the composition of these primordial black holes and potentially uncovering an entirely novel class of exotic black holes in the process. "It was really a delightful surprise," remarked David Kaiser, one of the study's authors.

"We were leveraging Stephen Hawking's renowned calculations concerning black holes, particularly his significant finding regarding the radiation emitted by black holes," Kaiser stated. "These exotic black holes emerge from our attempts to address the dark matter conundrum—they are an unexpected consequence of elucidating dark matter."

Scientists have proposed numerous hypotheses regarding the nature of dark matter, from unknown particles to additional dimensions. However, Hawking's black hole theory only recently gained traction. "It wasn't taken seriously until about a decade ago," said Elba Alonso-Monsalve, a co-author of the study and an MIT graduate student. "This was because black holes once seemed incredibly elusive—in the early 20th century, they were considered merely mathematical curiosities, devoid of any physical existence."

We now understand that nearly every galaxy hosts a black hole at its core, and the discovery of gravitational waves generated by colliding black holes in 2015—a groundbreaking revelation—demonstrated their ubiquity. "In fact, the universe is teeming with black holes," Alonso-Monsalve stated. "Yet, the dark matter particle remains elusive, despite extensive searches in the locations where it was anticipated. This does not imply that dark matter is not a particle, or that it is definitely black holes. It could be a combination of both. However, black holes as contenders for dark matter are now considered with much greater seriousness."

Other recent studies have corroborated the validity of Hawking's hypothesis, but the work of Alonso-Monsalve and Kaiser, a physics professor and the Germeshausen Professor of the History of Science at MIT, delves deeper, examining the precise circumstances surrounding the formation of primordial black holes. The study, published on June 6 in the journal Physical Review Letters, reveals that these black holes must have emerged within the first quintillionth of a second following the Big Bang: "That is truly early, and significantly earlier than the moment when protons and neutrons, the particles that constitute everything, were formed," Alonso-Monsalve said. In our everyday world, we cannot find protons and neutrons separated; they function as fundamental particles.

However, we recognize they are not, as they are composed of even smaller particles known as quarks, bound together by other particles called gluons. "You cannot find quarks and gluons unbound and free in the universe today, because it is too cold," Alonso-Monsalve added. "But in the early stages of the Big Bang, when it was extremely hot, they could be found unbound and free. Thus, primordial black holes formed by absorbing free quarks and gluons." Such a formation would render them fundamentally distinct from the astrophysical black holes typically observed in the universe, which result from collapsing stars. Additionally, a primordial black hole would be considerably smaller—on average, the mass of an asteroid, condensed into the volume of a single atom. Yet, if a sufficient number of these primordial black holes did not evaporate in the early stages of the Big Bang and persisted to the present day, they could account for all or most of the dark matter.

During the formation of primordial black holes, another type of previously unseen black hole must have formed as a kind of byproduct, according to the study. These would have been even smaller—merely the mass of a rhinoceros, condensed into less than the volume of a single proton. These minuscule black holes, due to their diminutive size, would have been able to acquire a rare and exotic property from the quark-gluon plasma in which they formed, known as a "color charge."

This state of charge is exclusive to quarks and gluons, never found in ordinary objects, Kaiser stated. This color charge would make them unique among black holes, which typically possess no charge whatsoever. "It's inevitable that these even smaller black holes would have also formed, as a byproduct of primordial black holes' formation," Alonso-Monsalve said, "but they would not exist today, as they would have already evaporated."

However, if they persisted for just ten millionths of a second into the Big Bang, when protons and neutrons formed, they could have left observable traces by altering the balance between the two particle types. "The balance of how many protons and how many neutrons were created is very delicate and depends on what other substances existed in the universe at that time. If these black holes with color charge were still present, they could have shifted the balance between protons and neutrons (in favor of one or the other), just enough that in the next few years, we could measure that," she added. The measurement could originate from Earth-based telescopes or sensitive instruments on orbiting satellites, Kaiser stated. But there could be another method of confirming the existence of these exotic black holes, he added.

"Creating a population of black holes is an extremely violent process that would send enormous ripples in the surrounding space-time. Those would be diminished over cosmic history, but not to zero," Kaiser said. "The next generation of gravitational detectors could catch a glimpse of the small-mass black holes—an exotic state of matter that was an unexpected byproduct of the more mundane black holes that could explain dark matter today."

What does this imply for ongoing experiments attempting to detect dark matter, such as the LZ Dark Matter Experiment in South Dakota? "The notion that there are exotic new particles remains an intriguing hypothesis," Kaiser said. "There are other types of large-scale experiments, some of which are under construction, searching for innovative ways to detect gravitational waves. And those indeed might pick up some of the stray signals from the extremely violent formation process of primordial black holes."

There's also the possibility that primordial black holes constitute only a portion of the dark matter, Alonso-Monsalve added. "It doesn't necessarily have to be all the same," she said. "There is five times more dark matter than regular matter, and regular matter is composed of a multitude of different particles. So why should dark matter be a single type of object?" Primordial black holes have regained popularity with the discovery of gravitational waves, yet not much is known about their formation, according to Nico Cappelluti, an assistant professor in the physics department of the University of Miami.

He was not involved with the study. "This work is an intriguing, viable option for elucidating the enigmatic dark matter," Cappelluti said. The study is thrilling and proposes a novel 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 was also not involved with the study. "All the hydrogen and helium that we have in our universe today was created in the first three minutes, and if enough of these primordial black holes were present until then, they would have influenced that process and those effects may be detectable," Natarajan stated. "The fact that this is an observationally testable hypothesis is what I find genuinely exhilarating, aside from the fact that this suggests nature likely creates black holes from the earliest times through multiple pathways."