More than two kilometers below the surface of northern Ontario, suspended in 345,000 liters of ultra-pure water, there’s a perfect sphere. It contains 3600 kilograms of liquid argon, cooled to -180 degrees Celsius. Scientists continuously monitor this chamber from above ground, looking for a glimmer of light in the darkness.
Because down here, deep beneath the Earth’s surface and cocooned in a watery shield, that light would indicate the presence of one of the universe’s greatest mysteries: dark matter. All the matter we can see, planets, stars and galaxies, doesn’t create enough gravitational pull to explain the universe’s larger structure. It’s dark matter, which is estimated to make up 25% of the known universe.
But despite its prevalence, so far we haven’t been able to detect it directly. It’s no small challenge. Dark matter was so named because it doesn’t interact with any type of light, visible or otherwise, which means our usual observation tools simply don’t work when trying to observe it.
But while dark matter may not be visible in the electromagnetic spectrum, it’s still matter, so we should be able to measure its interactions with other matter. And if our current model of physics is correct, billions of sub-atomic dark matter particles are passing through the Earth every second. Despite the prevalence of dark matter, its interactions are predicted to be rare and extremely weak. To detect these interactions, dark matter experiments need to be incredibly sensitive.
With such sensitive equipment, the ever-present background radiation on Earth’s surface would create so much noise in the data that any dark matter particles would be completely overwhelmed. It would be like trying to hear a pin drop on a busy city street. To solve this problem, scientists have had to dig deep into the Earth. Dark matter experiments are set up in specialized underground labs, either in mines or inside mountains. The rock that makes up the Earth’s crust works like a filter, absorbing radiation and stopping disruptive particles.
The ultra-pure water in which the detector is suspended adds an additional layer of radiation filtering. This shielding ensures that only the particles scientists are looking for can make their way into the detectors. Once these particles reach an experiment’s inner vessel, scientists have a chance of detecting them.
The detector media are chosen because they’re exquisitely sensitive detectors that can be purified extremely well. These could be a liquid noble gas, germanium and silicon crystals, a refrigerant, or other materials. When radiation interacts, it leaves tell-tale signs, such as light or bubbles, which can be picked up by the sensors inside the detector.
The detector media are held in a central chamber made of glass or a special type of acrylic. These chambers have to be able to hold the substance inside without interacting with it while withstanding incredible pressure from the water outside. The inner vessel is surrounded by powerful sensors designed to detect even the tiniest blips of light, or the sound vibrations caused by a single bubble.
Each sensor records data 24/7, and experiments run for months and years at a time, generating terabytes of data every day. Building dark matter detectors is as much a feat of engineering as it is a feat of physics. By the time an experiment is ready to start collecting data, years or decades of work and investment have already gone into it, to the tune of tens of millions of dollars. As of 2017, no dark matter particles have been directly detected. That’s not entirely surprising.
Physicists expect these interactions to be incredibly rare and difficult to detect. In the meantime, scientists continue to develop new technologies and increase detector sensitivity, closing in on where dark matter is hiding. And when they find it, we’ll finally be able to bring the universe’s darkest secrets into the light.