Not long ago, the actor Tilda Swinton—cosmic muse to cinéastes, fashion designers, and physicists—took on another shape-shifting role as the voice of a new a planetarium film, “Phantom of the Universe: The Hunt for Dark Matter.” “As we look out into the night sky, we are both dazzled and comforted by the patches of light we find there,” her narration begins. In time, Swinton continues, astronomers started to suspect that there was something more out there than these brilliant moons, stars, and galaxies—“something hiding in the dark spaces.” The film premièred in Mexico City, on Sunday, and today has special showings worldwide in celebration of this, the inaugural Dark Matter Day.
Everything that humans have seen up until now exists in the 4.9 per cent of the universe that interacts with light. The rest is hidden from view. Most of it, physicists believe—68.3 per cent—is dark energy, an enigmatic force that drives the accelerating expansion of the cosmos. The rest—26.8 per cent—consists of dark matter, a ghostly goo that is thought to hold the cosmos together. This is why the Interactions Collaboration, a global consortium of particle-physics laboratories, has reimagined Halloween as Dark Matter Day. “Dark matter seems to ‘hide’ in plain sight and doesn’t play by the known rules of physics,” a promotional F.A.Q. explains. “It’s like a costumed trick-or-treater who rings the doorbell and then dashes away, and scientists are trying to unmask it!”
Dark matter was first theorized, in the nineteen-thirties, by Caltech’s Fritz Zwicky, who reputedly referred to his colleagues at the Mount Wilson Observatory, in Los Angeles, as “spherical bastards,” since he found them equally disagreeable from all sides. (Costume idea!) Forty years later, Vera Rubin, of the Carnegie Institution for Science, in Washington, D.C., confirmed Zwicky’s theory. Studying the rotation of galaxies, Rubin and her collaborators observed that, given the galaxies’ spiralling speeds, and given their visible mass, these stable structures should in fact be flying apart. This amounted to circumstantial evidence that an invisible incarnation of matter—a halo, as it’s occasionally called—kept them whole.
Now thousands of physicists have joined the hunt. But looking for the subatomic source of dark matter—the leading candidate is known as the WIMP, for weakly interacting massive particle—has proved an expensive and frustrating, if occasionally edifying, odyssey. At the European Organization for Nuclear Research (CERN), in Switzerland, where Dark Matter Day will be celebrated with Dark Matter Cake—baked with the cosmically correct proportions of white-chocolate chips (visible matter), dark-chocolate chips (dark matter), and beetroot (dark energy)—the universe’s mystery ingredient is “definitely in the spotlight now,” Oliver Buchmueller, a senior physicist at Imperial College London, told me. Now that the Higgs boson is well accounted for, dark matter has become one of the Large Hadron Collider’s main targets. The favored model for predicting dark matter has long been supersymmetry. As Swinton explains in the film, “According to this theory, for every known particle, like an electron or quark, there’s a corresponding superparticle with a much greater mass.” But since none of these partners have shown themselves at the L.H.C., researchers are now testing more generic scenarios. “One hypothesis is that the Higgs could be a portal connecting us to the dark world,” Buchmueller said. “We know that the Higgs boson gives mass to all our fundamental particles. But, instead of just decaying to these particles we know in the visible world, the Higgs might also decay to the dark-matter particles.” So far, though, no dice.
The same is true at SNOLAB, in Sudbury, Ontario, a facility buried more than a mile underground, in an active nickel-and-copper mine. Here the goal is to actually observe dark-matter particles as they pass through the planet; the overlying rock filters out the noisy cosmic rays that would otherwise smother the signal. On a visit in August of last year, I joined a group of miners aboard the 8:05 A.M. cage, travelling downward at twenty-five miles per hour. (Thankfully, I didn’t faint; enough SNOLAB visitors do that the miners call them SNOflakes.) I was there to see the DEAP-3600 liquid-argon detector, which, after six years in the making, was just about to become operational. The ultra-pure argon, cooled to minus 297 degrees Fahrenheit, is meant to serve as a conduit of sorts, lighting up in the presence of dark matter. Almost a year later, in July, 2017, the DEAP team published its first results: “No candidate particles are observed.” As Richard Gaitskell, the spokesman for the equally unsuccessful Large Underground Xenon detector, in the Black Hills of South Dakota, told me, “So far we’ve always gotten a negative result. Which means we only know what dark matter isn’t.” The upshot, he said, is that “there are basically thousands of models of particle physics lying bloodied in the gutter.”
Along the way, however, there have been consolation prizes. In September, 2009, M.I.T.’s Tracy Slatyer and her colleagues analyzed new data from the Fermi Gamma-Ray Space Telescope and spotted a fuzzy blob of gamma rays extending far above and below the core of the Milky Way galaxy. Could this be a relic of dark matter, they wondered? Alas, it turned out to be something else—“Just something we hadn’t dreamed of yet,” Slatyer said: a figure-eight-shaped pair of bubbles, likely an eruption from a black hole five million times as massive as our sun.
In examining these so-called Fermi bubbles, Slatyer and another team noticed something more: an excess of gamma rays emanating from the galactic center. “We believe that dark matter piles up in the center of galaxies, because it’s pulled there by gravity,” she told me. That makes the galactic center a good place to look, though also a frightening place, she noted, because it’s populated by so many violent and high-energy astrophysical phenomena. The gamma-ray excess could come from dark matter, or it could come from a population of rare millisecond pulsars—city-sized neutron stars spinning around at a rate of a thousand times per second. Slatyer is ninety-five per cent confident that this is another false alarm. (Her more optimistic colleague gives dark matter a fifty-fifty chance.) But, Slatyer said, “the best thing about these false alarms in astrophysical data is that even if they turn out not to be dark matter, they often tell you about something very interesting. You get a discovery either way.”
Perhaps the most pessimistic proposition involves the recent revival of a radical theory from the nineteen-eighties known as MOND, or modified Newtonian dynamics, which hypothesizes that there is no dark matter—none at all. Rather, the galactic conundrum is solved by a shift in our understanding of gravity. “When I was a kid, I would wake up one night out of every thirty and think, Oh, my God! It’s probably MOND!” Nima Arkani-Hamed, of the Institute for Advanced Study, in Princeton, told me. “And the other twenty-nine nights, I would be happy that it was probably dark matter. Then I became a scientist, and now it’s once a year that I’ll look up and be, like, Oh, my God. Maybe it’s MOND. But I don’t think it is. It doesn’t smell right to me.”
But, then again, the worst-case scenario is that, in ten years, or a hundred, this spooky predicament remains a mystery. Sure, the joy is in the hunt—and, as Swinton concludes in her narration, “Ultimately, it’s the big questions that bring humankind together”—but to spend lifetimes searching for something and not finding it would be, well, astronomically frustrating. “The problem is, we have no idea what we are looking for,” Hugh Lippincott, of the Fermi National Accelerator Laboratory, outside Chicago, said. “And there is a not insignificant chance, probably better than fifty per cent, that we are never going to find it. That’s the scary part.”
