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Science and Discovery

Searching for dark matter

Most of the mass of the universe is missing. Where is it and how do we know it exists? Scientists at SURF think WIMPs could be the answer.
Dark matter universe visualization
The universe in a pie chart.

A slice of the universal pie

Everyone on Earth, and all of the planets, stars and intergalactic gases make up just a fraction—about 5 percent—of all the matter in the universe. So, where’s the rest? Dark energy makes up about 68 percent of the universe; the remaining 27 percent is dark matter. We call it dark matter simply because we don't know what it is and we can't see or touch it. 

So, how do we know dark matter exists? We can see its affects on galaxies and clusters of galaxies. Without dark matter, galaxies don't have enough mass to stay together—they would fly apart. There is also indirect evidence that hints at its existence. But scientists underground at Sanford Lab are looking to directly detect dark matter.

A gravitational lens mirage

Indirect evidence

We don't know much about dark matter yet, which is a remarkable thing because it makes up 80-85 percent of all the matter in the entire universe. We've never been able to directly detect dark matter in any form, but we know it exists through its effects on the universe, especially through the orbital velocities of stars and gravitational lensing of light around "invisible" objects.

Gravity keeps planets rotating around the sun and the solar system rotating around the galaxy. But when we observe the speed at which galaxies rotate, there simply isn't enough gravity to hold everything together. Think of dark matter as the glue that allows galaxies to generate the extra mass and gravity to keep everything together.

Gravitational lensing occurs as light bends around massive objects such as galaxies, clusters of galaxies and even our own sun. It has been observed for more than a hundred years but becomes especially interesting when observed where there is nothing visible to bend the light.

Dark matter streams

The case for WIMPs

Researchers at the Sanford Underground Research Facility believe the leading candidate for a dark matter particle is a WIMP, or Weakly Interacting Massive Particle. Billions of WIMPSS pass through uss, the Earth and everything on it every second. But because WIMPs interact so weakly with ordinary matter, their ghostly journey goes entirely unnoticed.

Scientists hope to see a WIMP interact with normal matter through the weak nuclear force. At SURF they constructed the LUX-ZEPLIN dark matter detector and filled it with 10 tons of cooled super-dense liquid xenon. The xenon is surrounded by powerful sensors designed to detect the tiny flash of light and electrical charge emitted if a WIMP collides with a xenon atom within the tank. The detector’s location nearly a mile of rock, and inside a 72,000-gallon, high-purity water tank, helps shield it from cosmic rays and other radiation that would interfere with a dark matter signal.

Temperature to turn Xenon to liquid

Our target is xenon, a colorless, odorless noble gas found in trace amounts in the Earth’s atmosphere. In the search for WIMPs, scientists with LUX and LZ use super dense liquid xenon, which is cooled by liquid nitrogen to -160 Fahrenheit. We use xenon for its ability to emit light and electrons when hit by other particles—a property critical to detecting WIMPs.  

Amount of xenon in the second generation experiment

The first-generation dark matter detector, LUX, used 350 kilograms of xenon as a target. Now scientists have gone bigger—30 times bigger, in fact. LUX-ZEPLIN (LZ), holds 10 metric tons of xenon—nearly ¼ of the all xenon produced in an entire year. 

SURF purchased about 80 percent of the xenon needed for LZ. To ensurre the xenon was xenon was pure enough for the experiment, scientists ran it through a special purification process at SLAC National Accelerator Lab. In 2022, after 5 years of construction and installation, LZ published its first results. In 2023, the experiment became the most sensitive dark matter detector on Earth.

LZ particle detection graphic

Liquid xenon WIMP detectors

LZ uses a cryostat surrounded by tank filled with liquid scintilllator to hold the liquid xenon. Arrays of photomultiplier tubes, or PMTs, sit above and below the xenon target, where they create an electrical field. Scientists install the detector deep underground to reduce the noise of cosmic rays by a factor of about 10 million. Then they wait, and wait, hoping a WIMP will interact inside the detector.

How will they know they've seen a dark matter particle? Scientists believe that when a WIMP collides with a xenon atom, it creates a flash of light, releasing electrons. The PMTs see that and the electrons are noted by the electrical field. If this happens at the very center of the detector it just might be the signal scientists hope to see.