When an incoming particle strikes an atomic nucleus, it can lead to the production of free charges and/or photons, which can produce a signal visible in the photomultiplier tubes surrounding the target. The XENON detector leverages this idea spectacularly, making it the world’s most sensitive particle detection experiment. (Credit: Nicolle Rager Fuller/NSF/IceCube)

XENON’s experimental triumph: no dark matter, but the best “null result” in history

Searching for dark matter, the XENON collaboration found absolutely nothing out of the ordinary. Here’s why that’s an extraordinary feat.

More than 100 years ago, the foundations of physics were thrown into utter chaos by an experiment that measured absolutely nothing at all. Knowing that the Earth moved through space as it rotated on its axis and orbited the Sun, scientists sent beams of light in two different directions — one along the Earth’s direction of motion, and one perpendicular to it — and then reflected them back to their starting point, recombining them upon arrival. Whatever shift the Earth’s motion would have caused within that light would be imprinted on the recombined signal, allowing us to determine what the true “rest frame” of the Universe was.

And yet, there was absolutely no shift observed at all. The Michelson-Morley experiment, despite achieving a “null result,” would wind up transforming our understanding of motion within the Universe, leading to the Lorentz transformations and special relativity thereafter. Only by achieving such a high-quality, high-precision result could we learn what the Universe was and wasn’t doing.

Today, we understand how light travels, but other, more difficult-to-solve puzzles remain, like figuring out the nature of dark matter. With their latest, greatest results, the XENON collaboration broke their own record for sensitivity to how dark matter could possibly be interacting with atom-based matter. Despite a “null result,” it’s one of the most exciting results in experimental physics history. Here’s the science of why.

The dark matter structures which form in the Universe (left) and the visible galactic structures that result (right) are shown from top-down in a cold, warm, and hot dark matter Universe. From the observations we have, at least 98%+ of the dark matter must be either cold or warm; hot is ruled out. Observations of many different aspects of the Universe on a variety of different scales all point, indirectly, to the existence of dark matter. (Credit: ITP, University of Zurich)

Indirectly, the evidence for dark matter comes from astrophysically observing the Universe, and is absolutely overwhelming. Because we know how gravitation works, we can calculate how much matter needs to be present in various structures — individual galaxies, in pairs of interacting galaxies, within clusters of galaxies, distributed throughout the cosmic web, etc. — to explain the…