Ever since astronomers reached a consensus in the 1980s that most of the mass in the universe is invisible—that “dark matter” must glue galaxies together and gravitationally sculpt the cosmos as a whole—experimentalists have hunted for the nonluminous particles.
They first set out in pursuit of a heavy, sluggish form of dark matter called a weakly interacting massive particle, or WIMP—the early favorite candidate for the cosmos’s missing matter because it could solve another, unrelated puzzle in particle physics. Over the decades, teams of physicists set up ever larger targets, in the form of huge crystals and multi-ton vats of exotic liquids, hoping to catch the rare jiggle of an atom when a WIMP banged into it.
But these detectors have stayed quiet, and physicists are increasingly contemplating a broader spectrum of possibilities. On the heavy end, they say the universe’s invisible matter could clump into black holes as heavy as stars. At the other extreme, dark matter could spread out in a fine mist of particles thousands of trillions of trillions of times lighter than electrons.
With new hypotheses come new detection methods. Kathryn Zurek, a theoretical physicist at the California Institute of Technology, said that if current WIMP experiments don’t see anything, “then I think there’s going to be a substantial part of the field that’s going to shift into these new kinds of experiments.”
Already, the work has begun. Here are a few of the many new fronts in the search for dark matter.
Between an Electron and a Proton
WIMPs would have enough heft to occasionally bowl over a whole atom. But in case dark matter is lighter, some experimentalists are setting up smaller bowling pins.
A gentler rain of dark matter particles weighing less than protons could occasionally knock electrons free from their host atoms. The first experiment designed specifically to pick up this dark matter is the Sub-Electron-Noise Skipper CCD Experimental Instrument (Sensei), which uses technology similar to that of digital cameras to amplify signals from unexpectedly emancipated electrons inside materials.
When a Sensei prototype switched on with just one-tenth of a gram of silicon, it didn’t find dark matter. Even so, the team’s results, published in 2018, instantly ruled out certain models.
“We just turned on and we had the world’s best limits,” said Tien-Tien Yu, a physicist at the University of Oregon and a Sensei collaboration member, “because there were no limits before.”
Recent results from a 2-gram version of Sensei extend those limits, and now Yu and her colleagues are preparing to deploy a 10-gram version in an underground laboratory in Canada, away from interfering cosmic rays. Other groups are designing alternative low-cost experiments targeting the same low-hanging fruit.
If dark matter is lighter still, or blind to electric charge, it might fail to unleash an electron. Zurek has brainstormed ways that even these pipsqueaks could betray their presence by influencing the behavior of groups of particles.
Imagine a block of silicon, for example, as a mattress with springs representing atomic nuclei. Bounce a quarter off the mattress, Zurek says, and while no single spring will move much, the coin could set off a ripple that passes through many springs. She proposed in 2017 that an analogous disturbance from a dark matter interaction might generate sound waves that could slightly warm the system.
One project taking this route, Tesseract, is currently running in a basement at the University of California, Berkeley, looking for ripples from dark particles similar in heft to those that Sensei targets. Sensitive future upgrades, however, could theoretically find particles up to a thousand times lighter.
But there are even more Lilliputian particle possibilities. The axion—an entity so slight it’s more wave than particle—could comprise dark matter and simultaneously solve a mystery about the strong nuclear force. The Axion Dark Matter Experiment (ADMX) recently began scanning for axions decaying into pairs of photons inside a mighty magnetic field, and several similar searches are starting up.
Some experiments are aiming for even lighter stuff. The lightest that dark matter could possibly be is about one-thousandth of a trillionth of a trillionth of the electron’s mass—which would result in a particle that’s like an extremely low-energy wave, with a wavelength the size of a small galaxy. Lighter (and therefore longer) entities would be too diffuse to explain why galaxies stick together.
Clues From Above
While experimentalists prepare the next generation of apparatuses seeking direct contact with dark matter, others plan to scour the heavens for indirect signposts.