Past a bridge stretching over the Yalong River in the Jinping Mountains of Sichuan, China, dozens of the most talented physicists from around the world drive ten miles down a deep tunnel each workday. Near the end of their journey, approximately 7,900 feet underground, they see a faint light illuminating a huge white building with tiled walls and a blue logo plastered to the top. They’ve reached the China Jinping Underground Laboratory.
Here, day and night become meaningless, and the only signs of life are the scientists, themselves, who are feverishly hunting for dark matter—the invisible, mysterious form of matter that hypothetically constitutes 85 percent of our universe; scientists only know it exists due to its gravitational pull. Since it was inaugurated in 2010, the China Jinping Underground Laboratory (CJUL) has set records for being the largest earthbound venue for dark matter detection thus far, though it only became operational in December 2023.
In searching for something that might not even exist, the concept of dark matter tests the patience and creativity of particle physicists and cosmologists around the world. Dark matter is the most “explicit anomaly” of our universe, dark matter researcher Yeongduk Kim, Ph.D., tells Popular Mechanics. But despite multiple reports of dark matter detection popping up around the world in the last few decades, most ended up being false signals.
Kim, who has been searching for dark matter for over 30 years, is aware that it’s entirely possible he and his fellow researchers will have nothing to show for all their years of work. Regardless, scientists appear unfazed—if anything, that uncertainty in itself seems to be the gasoline igniting a race to uncover dark matter’s identity. That’s because everyone involved agrees on one thing: no matter what dark matter ends up being, the mere confirmation of its existence will rewrite the rules of the universe as we know it. For instance, the sheer abundance of dark matter in our universe could be theoretically used as a free, unlimited source of energy.
But how do you do that with particles that are still a mystery? “We have a very good and complete theory which is the Standard Model [of particle physics] that explains almost every question—every aspect of nature and matter, atoms, elementary particles,” Kim explains. “All phenomena except dark matter.”
“All we can say for sure is that these particles don’t interact very much, because if they did, we would have seen them already,” says Daniel Hooper, Ph.D., a cosmologist at the Batavia, Illinois-based particle accelerator Fermilab and the University of Chicago. “If they have any electric charge, for example, we would have already detected the light from them, either the light they’d create or maybe light that they would absorb,” he tells Popular Mechanics.
To find the truth about this elusive dark matter, cutting-edge research is underway nearly a mile and a half beneath Earth’s surface.
As the deepest detector in the world, the CJPL takes advantage of the thousands of feet of rock between its sensitive instruments and the surface. Only one in 1 million cosmic rays from the sun that constantly bombard our planet can penetrate this cavernous space, making it possible to study dark matter.
A virtual look at a researcher’s journey into the underground China Jinping Underground Lab
Powered by hydroelectric generators, CJPL’s dark matter search consists of two research projects: Particle and Astrophysical Xenon Experiments (PandaX) and the China Dark Matter Experiment (CDEX), two large tanks of xenon and germanium, respectively, sitting in complete isolation from the rest of the world. Both PandaX (pictured at the top of this story) and CDEX employ methods for “direct detection” of dark matter. In particular, they’re seeking out weakly interacting massive particles (or WIMPs), a type of hypothetical, heavy, and slow-moving subatomic particle that barely interacts with ordinary matter.
Scientists turn to gases xenon and germanium as the best candidates for interaction with dark matter particles. Neither of these elements react easily with other matter, so any interactions that do occur between them and WIMPs are more likely to be due to the presence of WIMPs. What makes CJPL unique is its use of both xenon and germanium; competing facilities, such as LUX-ZEPLIN in South Dakota or Italy’s XENONnT, operate solely on xenon detectors.
“PandaX has a real opportunity in the future to become not just a world leader, but the world leader,” Jonathan Ellis, Ph.D., theoretical physicist and chairman of the CJPL advisory committee, tells Popular Mechanics. CDEX is a bit more complicated, he says. That’s because germanium is more tricky to handle.
Compared to xenon, germanium is not only more expensive, but also more difficult to scale up in mass and sensitivity, Ellis explains. Nevertheless, germanium detectors are capable of identifying what traditional xenon detectors cannot. “Xenon is very suitable for looking for dark matter particles weighing dozens of times a mass of the proton,” he says, “whereas the germanium material that CDEX uses is better suited for the lighter part of WIMPs.”
🐞 The scale these scientists work on is extremely tiny: about one quadrillionth of one nanogram. That’s one billion times smaller than one gram, which is the average weight of a paper clip.
Liquid xenon is ideal for hunting WIMPs because it’s a relatively stable element that doesn’t allow for extra noise to enter, while being heavy or dense enough for WIMPs to “leave a mark.” Xenon detectors have the capacity to detect potential dark matter candidates almost twelve times smaller than the average Xenon nucleus. In that sense, PandaX and CDEX complement each other in terms of the lab’s detection capacity, Ellis says.
The reason direct detection facilities are so far underground is to block out external “noise” from the surface. In spite of the name “direct detection,” the detectors themselves don’t sound alarms upon discovering dark matter. Rather, dark matter passes through the detector all the time, but very rarely interacts with the xenon or germanium inside the tank.
“Imagine that you have a bullseye,” says Juan Collar, Ph.D., a particle physicist at the University of Chicago. “If it’s very small and you throw a projectile—a dart towards that bullseye—you have a very little chance of hitting the bullseye. If [it is] bigger, then the dart, the projectile, the probe that you sent might have a bigger chance of striking the bullseye,” he tells Popular Mechanics.
Physicists observing the detector essentially scour the signals the liquid tank records for the tiniest signs of a near-invisible disturbance, the result of a chance interaction between the detector and dark matter. By shielding the detector from identifiable cosmic rays, the goal is to zero in on unknown exotic disturbances.
Given the extreme sensitivity this process demands, it’s only natural that the search for dark matter by itself pushes the boundaries of astrophysics, Don Lincoln, Ph.D., a senior scientist at Fermilab, tells Popular Mechanics. However illogical dark matter research may seem, many of our “cosmic mysteries” start to make a lot of sense once dark matter comes into the picture.
For instance, assuming our theory of gravity is correct, the galaxies of our observable universe must rotate at a certain speed. In reality, galaxies not only spin around a lot faster than the math says they should, but also so fast that they should fly away from one another. However, the existing theories of physics can’t quite explain what could be holding them together.
“And the possible solution could be dark matter,” Lincoln says, “but an alternative explanation could be that we don’t understand the laws of motion, nor do we understand the law of gravity.”
Physicists have tried out the alternatives, but nothing thus far has connected the stars—literally—as well as the theory that there is “more matter than we can see,” in the universe, says Lincoln.
And the quest to illuminate the cosmic mysteries of our universe is mostly just that: closing old doors and opening new ones. An idea proven wrong just eliminates one extra distraction, just as digging deep underground to install detection technology eliminates one extra noise factor.
“So there are a lot of people out there who have become, I don’t know—deflated, maybe, by the fact that we haven’t discovered dark matter,” Hooper says, explaining that this fatigue or apprehension plays into an overarching, somewhat historical theme of fundamental physics.
“The analogy I like to use is, a few hundred years back, what did people know about air? It’s obvious the stuff is real, but we didn’t know it was made of diatomic nitrogen and oxygen and the smattering of other [particles],” he points out. “Like, that stuff was a mystery.”
When the electron was discovered, Hooper explains, many considered it to have “no inkling of any practical application.” Nonetheless, had we never discovered or studied electrons, we would be living without computers, superconductors, and most household electronics.
Astrophysicist Ethan Siegel, Ph.D., who hosts the podcast Starts With a Bang!, can foresee a future that uses dark matter particles as a kind of rocket fuel. All matter has its own antimatter equivalent, Siegel explains, and colliding matter with antimatter creates efficient “pure energy.” If dark matter really ends up having little to no charge, it would mean that each dark matter particle would behave as its own antiparticle. If this were true, slamming two dark matter particles together would result in a matter-antimatter explosion—a consistent source of perfectly efficient energy for rockets.
That’s not all we might see. “I wouldn’t be at all surprised if we had a time machine and went forward 1,000 years, and dark matter had been discovered, and it was being widely used in lawn mowers and vacuum cleaners,” Charlie Conroy, Ph.D., an astrophysicist at Harvard University, tells Popular Mechanics. In any case, he says, never underestimate the creativity of humans as a species—especially if the stakes are as grand as the cosmic mysteries dictating the rules of our entire universe.
There’s certainly a lot left to work out theoretically, Hooper says. Before we know dark matter could be rocket fuel, for example, we’ll need to figure out how dark matter—considered to be very weak-interacting particles—would move along with the rocket.
“And I will just say that sounds very, very hard,” he says. “If I’m using normal chemical fuel for a rocket, the reason I can do that is, I can apply force to that fuel like in a tank or something. But for all the reasons dark matter is, well, dark, it’s not easy to apply force to dark matter.”
Dark matter lawn mowers seem pretty unlikely to Kim, as dark matter doesn’t exist at the level of abundance as does the electron. But he too agrees that whatever we end up finding, we’ll find ourselves much closer to mapping out a clearer picture of how our universe evolved—and who knows what we’ll be able to do with that kind of knowledge?
“So I think we are, with our understanding of dark matter, in a similar place to where we were in 1700, when we were asking the question of what was air,” Hooper says. “It wasn’t a matter of figuring out if air existed, and it’s not a matter today of figuring out if dark matter exists. It’s a matter of figuring out what it consists of—not exists—but consists.”
Gayoung Lee is a science writer and illustrator from South Korea. A philosopher by training, her interests lie in uncovering and writing about the unexpected connections between the world and various scientific phenomena, particularly in theoretical physics and chemistry. You can learn more on her website.
