Massive underground laboratory in China joins the quest to find dark matter

Dark matter — an invisible substance that somehow makes up over 80 percent of all matter in the universe — is a phenomenon that frustratingly eludes scientists. Despite being sought after for decades, and providing us with clues that it does indeed exist, dark matter has never been directly detected.

But now, the China Jinping Underground Laboratory (CJPL) — crowned the world's largest and deepest underground facility after its upgraded phase, CJPL-II — promises to take scientists a step further. It became operational in early December of last year.

Built inside repurposed tunnels running through the Jinping Mountains in China's Sichuan Province, the lab is buried beneath 2,400 meters (1.49 miles) of rock. The reason for its deep, lonely location is that so much rock can reduce background noise found in dark matter data, typically induced by things like cosmic rays (another space mystery for another time.)

Related: How much of the universe is dark matter?

Spread across 330,000 cubic meters, the enormous new facility is home to two upgraded dark matter detectors. The lab also has exceptional horizontal access. "One can drive a bus to the caverns," said Wick Haxton, a professor of physics at the University of California, Berkeley, who has toured CJPL-II as well as the original laboratory, and was on the laboratory's advisory committee until last year. 

This impressive aspect "makes the construction of large facilities underground less costly and more efficient," Haxton said. "I do not believe there is any other site that combines such great depth with such access."

Detecting the invisible 

Scientists think the dark matter content that permeates our universe doesn't experience many of the interactions that charged particles, like protons and electrons, would, meaning particles thought to make up the mysterious substance could very well glide right through Earth's rock and pass through detectors located even below the surface at China's Jinping lab. 

After all, a key characteristic of dark matter is the fact that it doesn't interact with light, unlike "normal," or baryonic, matter composed of protons and electrons. That's actually why it's fully invisible to us.

In the lab, however, scientists hope potential dark matter particles collide with material in detectors designed to flag these elusive particles. At Jinping, this search is spearheaded by two dark matter experiments, named the Particle and Astrophysical Xenon Experiments (PandaX) and the China Dark Matter Experiment (CDEX). 

Potential dark matter particles colliding with atoms of liquid xenon maintained by the PandaX detector would be flagged by sensors as light flashes. Meanwhile, CDEX's higher-sensitivity germanium detector would tag these mysterious particles as electrical signals. The idea is that, even if nearly every dark matter particle whizzes past the detectors, at least one will accidentally come into contact with either of them.

Specifically, the detectors are hunting for a leading dark matter candidate called WIMPS (short for weakly interacting massive particles), a hypothetical class of particles predicted over three decades ago that have eluded the most sensitive experiments so far. Their presence, known only through the weak nuclear force and gravity, is within the understanding of how we think the universe evolved. In 2021, however, the world's most sensitive WIMP detectors at the Gran Sasso National Laboratory in Italy as well as Jinping reported a null finding.

Another leading alternative for the dark matter particle include axions, also a category of hypothetical particles thought to flood the universe and behave exactly like dark matter. Other exotic interpretations remain, such as the popular but unconfirmed theory that dark matter particles somehow interact with themselves (self-interacting dark matter or SIDM).

Dark matter experts say the upgraded Jinping lab could also help answer more fundamental questions, such as whether "particles" really constitute dark matter. A commonly held notion is that dark matter is primarily made of as yet undetected subatomic particle (or a group of them). But alternate popular (but imperfect theories) of gravity that don't require dark matter to be made up of particles persist.

"At a basic level, we don't know because we've never detected a particle interaction," said Matthew Walker, an astrophysicist at Carnegie Mellon University in Pittsburgh, Pennsylvania. As ambitious as the goal is, detecting a dark matter particle would settle the issue once and for all. "To me, that's the most important thing this experiment could do."

"I'm rooting for them," he added. "We've been waiting a long time to learn what dark matter is."