His team’s analysis, published in June in Monthly Notices of the Royal Astronomical Society, did not find evidence for wavelike dark matter effects in high-resolution images of arcs of light from one gravitational lens, suggesting that the dark particle must be heavier than the smallest fuzzy candidates. But an April study in Nature Astronomy, led by Alfred Amruth of the University of Hong Kong, looked at four lensed copies of a background quasar and came to the opposite conclusion: A lens made of fuzzy dark matter, they argued, better explained small fluctuations in their data. (Contradictory findings wouldn’t be entirely surprising given that the expected signals are subtle and the experimental approach is new, experts outside both teams tell Quanta.)

Nierenberg and her colleagues, meanwhile, have spent the last year using JWST to observe gravitational lenses that magnify quasars, with the tentative goal of publishing their first analysis in September. In theory, they calculate that JWST’s ability to uncover small-scale structure in lenses should reveal whether dark halos exist as fully invisible, starless clumps with a size range of tens of millions of solar masses. If so, those halos would impose the strongest constraint yet on how “warm” dark matter can be.

This even newer method of looking at extreme, faraway stars like Mothra through gravitational lenses might soon move from identifying one-off curiosities to becoming a regular feature of astronomy in the JWST era. If Diego and his colleagues are correct, and they can see Mothra because it’s being lensed by a dark matter clump weighing fewer than a few million solar masses, that observation alone would rule out a wide swath of warm dark matter models. But it would still support both cold and fuzzy dark matter, although in the latter case — where the extra magnification of Mothra comes from a dense granule of dark matter instead of a gravitationally bound clump — it would still force fuzzy dark matter into a narrow range of possible masses.

Astronomers are unearthing many more lensed stars with Hubble and JWST, Diego said, keeping an eye out for other anomalous optical distortions that could come from starlight bending around small dark objects. “We’re just starting to scratch the surface,” he said. “I don’t take much vacation these days.”

Dark Islands in a Stream of Stars

Other searches for small dark matter halos are focused on much closer stars — those in streamers near the Milky Way, and binary stars in nearby dwarf galaxies. In 2018, Ana Bonaca, now an astrophysicist at the Carnegie Observatories, raced to download data from the European Space Agency’s Gaia spacecraft, which measures the motions of nearly 2 billion stars in the Milky Way. Bonaca sorted through those initial observations and isolated the information from stars belonging to a structure called GD-1. What she saw was “immediately super exciting,” she said. “We rushed to write a paper in the next week or so.”

GD-1 is a stellar stream, a loose string of Milky Way stars that — if you could pick it out with the naked eye — would stretch more than halfway across the night sky. These stars were ejected from a globular star cluster long ago; they now orbit the Milky Way on either side of that cluster, bobbing behind and ahead of its path like buoys marking an interstellar channel.

In their analysis of GD-1, Bonaca’s team found the theoretical fingerprint of an interloping hunk of dark matter. Specifically, part of GD-1 seemed split in two as if a massive invisible object had blundered through the trail, pulling stars in its wake. That passing object, they calculated, may have been a dark matter sub-halo weighing a few million solar masses — making it, too, a contender for the smallest putative dark matter clump, and a potential threat to the toastier variants of warm dark matter.

But how to convert a single finding to something more statistical? By now, Bonaca said, astronomers have chronicled about 100 stellar streams. While only a handful have been studied in detail, each one that has been scrutinized has its own unusual kinks and bends that may come from gravitational encounters with similarly small, dark objects. But the observations aren’t conclusive yet.

“I think the best way forward is to analyze streams simultaneously,” she said, “to understand how much of [those unusual features] is coming from dark matter.”

On even smaller scales, Walker, at Carnegie Mellon, has spent the last year scanning JWST observations of dwarf galaxies in search of the most fragile star systems he can find: binary stars that are very far apart and held together in a loose gravitational embrace. If small dark halos — the sorts of objects that cold dark matter models say should be plentiful — are continually passing by and exerting gravitational pulls on their surroundings, these very wide binaries shouldn’t exist. But if wide binaries do show up, that suggests small dark halos aren’t present — striking a body blow against the many cold dark matter models that predict them.

“It’s what I call an anti-search for subgalactic dark matter halos,” Walker said.

Moving in the Walls

The search for cosmic shadows is still a small part of a larger effort to pin down something that has so far squirmed out of reach. Earth-based experiments designed to trap particles that would fit the fuzzy, warm and cold dark matter paradigms crank on; teams are still looking for other hallmarks of dark matter physics, from side products produced if and when the particles interact with normal matter, to the subtle question of how dark matter’s density rises and falls within dark halos, which depends on how the dark particles interact with each other.

Tracy Slatyer, a theoretical physicist at the Massachusetts Institute of Technology, visualizes the dark matter mystery as a vast box full of myriad possibilities but holding only one right answer. In this analogy, her strategy is to slice deep into that box with specific, disprovable ideas about the properties of dark matter particles. The sides of the box, though, represent the only true confining facts astronomers can provide, such as upper limits on how warm dark matter can be, and lower limits on how fuzzy — or lightweight — it can be.

If astronomers could confidently detect fully dark cosmic objects in the million-solar-mass range, that would be an “observational tour de force,” Slatyer said. “It would be incredible.” The walls of her box would move inward, shrinking the space available for possibilities.

Upcoming technology may soon transform these various searches from early stabs in the dark into deeper forays into the shadowy structures that undergird the universe. JWST will deepen its study of gravitational lenses in the coming years; Nierenberg’s group, for example, has started with eight such systems but plans to eventually analyze 31 of them. When it launches in 2027, the Nancy Grace Roman Space Telescope, a Hubble-grade observatory with a much wider field of view, should make it much easier to pan through dwarf galaxies as Walker is doing. The Vera C. Rubin Observatory, named for the pioneering astronomer whose observations forced researchers to take the mystery of dark matter seriously in the first place, will reveal more details of stellar streams once it starts observing from Chile in 2024. Together, the two observatories should turn up thousands of new gravitational lenses that can be scoured for dark substructures.

So far, none of the observations have toppled the popular cold dark matter models, which predict that the universe is littered with smaller and smaller clumps of the stuff. As astronomers continue the grueling work of combing for those clumps, many theorists and experimentalists hope that a particle physics experiment on Earth will cut to the heart of the mystery much faster.  But uncovering these isolated pockets of darkness — and any intricate physics that accompanies them — is like “getting a cleaner laboratory,” Slatyer said. “We’re at an exciting time.”