Explore inside the secret lab, where to find dark matter to change the perception of the universe

When Michelle Galloway drives to work, she will enter a tunnel to go deep into a mountain. At the end of the road, at the main entrance to the facility, the guards would always ask her for a secret password before allowing her through.

"Then this James Bond-style door would open in the rock," explains Galloway, a senior researcher at the University of Zurich. "It's so great."

Behind that door is the Gran Sasso National Laboratory in Italy. Located 1,400 meters above the ground, it is the largest underground laboratory in the world. And in one of these caves carved into the rock of the Apennine Mountains, there is a machine that could change humanity's understanding of the entire universe.

The bottom of the "time projection booth" is in the center of this image. There are 120 "eyes" scattered around the inside of this middle chamber, capturing light from interactions caused by subatomic particles called neutrons.

Galloway and her colleagues who conducted the XENONnT experiment. They all have the same goal in mind : Find out what dark matter is made of. Dark matter, whatever it is, is what makes up 85% of the total mass of the universe. It bends light and binds galaxies together, preventing them from splitting on their own. One such intriguing property alone is reason enough for physicists to want to know how it exists.

The remaining 15% - everything else in the universe, from Saturn's rings to the cells lining your stomach - is covered by the so-called Standard Model , the theory that describes all particles. essential material as we know it.

But dark matter makes it difficult for the Standard Model - because it doesn't fit. One idea, called supersymmetry, is that there are a host of other hard-to-detect particles that act as counterparts to those we already know. "If we find some support for supersymmetry, it would give us a way to extend the Standard Model," Galloway explains.

She and her colleagues hope to find the answer with the help of 8.6 tons of liquid xenon, a noble gas sometimes used as a general anesthetic. Galloway said: "It's extremely rare, so it's expensive. The last time the team bought them for about 12 euros per liter. At that price, 8.6 tons would have cost around 17 million euros - but it was acquired gradually and can be recycled."

About five tons of xenon, kept at -100 degrees Celsius, is pumped inside the facility's smallest of three chambers, with state-of-the-art detectors, which recently received a major upgrade. This inner compartment is called the time projection chamber, or TPC . It is designed to pick up the extremely faint signals of dark matter particles passing by the Earth. One "theoretical candidate" particle the team hopes to detect is called a weakly interacting mass particle - or "WIMP" for short. Galloway says that XENONnT is trying to catch a "WIMP wind" flying through space.

A worker walks carefully on the floor of the outer chamber, while wearing a special suit to avoid contaminating the equipment. The main detector is outside the white sheet of paper at the top. When in operation, this entire space will be filled with water.

If everything works as planned, a WIMP will enter the cylindrical TPC and strike the nucleus of the xenon atom, causing a small amount of light to escape. In this "atomic recoil" event, some electrons will also be released from the xenon. They will go to the top of the TPC and cause the subsequent emission of light when interacting with a layer of xenon gas.

The problem is, the light detectors used in the experiment can pick up all kinds of physical interactions, including background radiation, which is not evidence of dark matter. But by pinpointing the locations of the glowing events, the team was able to plot exactly where they occurred. And if many interactions at the right energy level have occurred at the center of the TPC, right in the middle of that bunch of liquid xenon, the researchers can be confident that these interactions are caused by WIMP.

Noise cancellation is one of the big challenges of an experiment like this. Xenon must be cleaned continuously, in order to separate the substances that naturally accumulate in it from the materials in the detector. The two outer chambers (pictured above) are filled with a special saline solution that slows the passage of particles, which can disrupt WIMP detection. And additional light-sensing devices in these outer compartments detect non-WIMP interactions so they can be mitigated. Think of it the same way you try to hear a baby bird chirping in a windy forest. If you can somehow block out the background noise and listen very carefully, you have a better chance of hearing it.

But, it is possible that dark matter is not made of WIMP. It could be a mixture of different particles, or something completely different. Still, the XENONnT experiment will bring us closer to the answer, one way or another. At the very least, the whole project is a reminder of how little we still know about the universe.

"Even if we didn't find out what it was all our lives," Galloway said. "It will still give us a certain perspective, I think."