LZ Mine
An illustration showing the LUX-ZEPLIN experiment, which will be located one mile underground at the Sanford Underground Research Facility in South Dakota, in a cavern within the former Homestake gold mine. SLAC National Accelerator Laboratory

What is dark matter? It is believed to make up nearly 85 percent of the observable universe and yet, we know absolutely nothing about what makes up this mysterious substance.

Particle physicists looking for dark matter mostly fall into one of the two camps — one that believes that it is made up of Weakly Interacting Massive Particles (WIMPs), which are heavy particles that interact with normal matter only through gravity and the weak nuclear force; and the other that thinks axions — a class of extremely light particles whose existence is predicted by an extension of quantum chromodynamics, which is a theory that lies within the ambit of the Standard Model of particle physics — are dark matter’s constituent particles.

Read: Black Hole Mergers May Yield Clues About Dark Matter

The LUX-ZEPLIN experiment, being built in a one-mile-deep abandoned gold mine at the Sanford Underground Research Facility in South Dakota, is a WIMP detector. Researchers from the University of Wisconsin-Madison who are involved in designing and testing the detector, revealed that the $55 million project has entered the final stages of assembly and construction, Thursday.

“The researchers’ goal is to take the experiment online as quickly as possible to compete in a global race to be the first to detect dark matter,” the university said in a statement.

If the WIMP idea is correct, billions of these particles should be passing through the Earth every second. The researchers associated with the LUX-ZEPLIN experiment hope that that every now and then, one of these particles would collide with atoms of liquid xenon — 10 tons of which will be filled in a massive underground chamber.

“The detector is set up like an enormous bell capable of ringing in response to the lightest tap from a dark matter particle,” the university explained in the statement. “If a piece of dark matter runs into a xenon atom, the xenon will collide with its neighbors, producing a burst of ultraviolet light and releasing electrons. Moments later, the free electrons will excite the xenon gas at the top of the chamber and release a second, brighter burst of light.”

Detector schematic
The heart of the LZ detector will be a 5-foot-tall time projection chamber (TPC) filled with 10 tons of liquid xenon. Hopes are that hypothetical dark matter particles will produce flashes of light as they traverse the detector. SLAC National Accelerator Laboratory

If LUX-ZEPLIN, scheduled to become operational in 2020, detects such flashes, it would be the first experimental evidence of interactions between WIMPs and ordinary matter.

If, on the other hand, dark matter is made up of axions, experiments like the advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) could play a key role in their detection — as outlined in a recent study published in the journal Physical Review D.

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According to the study, if these hypothetical particles have the predicted mass, spinning and colliding black holes should produce a cloud of axions — much like the cloud of electrons around an atom’s nucleus — due to a phenomenon known as superradiance. This should, in turn, produce gravitational waves, that can, in theory, be detected by LIGO.