Touch something. Anything. What do you feel? Does it push back? That’s electromagnetism. Any time you touch something solid, the sensation of not being able to put your hand right through it is caused by the electromagnetic force of the atoms in your hand pushing against the atoms of the object in question. Electromagnetism governs every interaction we have with the world—touch, sight, even smell and taste when you consider that the chemical reactions we perceive through these senses are changes in (electromagnetic) molecular bonds.
How strange, then, that the evolution of the universe, the motions of galaxies, the formation of massive objects on the largest scales, are all governed by something that appears to have no interaction with electromagnetism at all. Dark matter, the mysterious substance that makes up more than 80 percent of the matter in the universe, is literally untouchable. It passes right through you, through the planet, through stars and gas and everything in the universe that we can see. It is invisible, at the most fundamental level. But it does have gravity, which is the defining property of things we call matter. And it is so abundant that “luminous” matter—the stuff we can see and touch—is little more than an afterthought in the large-scale structure of the cosmos.
Astronomers have known about dark matter since the 1930s, and they’ve been seeking an explanation ever since. The first clues came from the mysterious motions of galaxies. Like a poltergeist, dark matter makes things fly around in ways they really aren’t supposed to. Stars orbiting the centers of their galaxies travel so fast that they should be flung out into the cosmos, but they aren’t. Galaxies in clusters dance around one another, held in formation by something unseen. We study astronomical images, carefully accounting for the gravitational pull of everything we can see—stars, gas, dust, even black holes lit up by the matter they’re consuming—and it simply isn’t enough. There must be something else there, a ghostly cloud of invisible matter surrounding and enveloping each galaxy and cluster. At a loss as to what this invisible substance could be, we call it dark matter.
The mystery of dark matter’s true identity lies at the intersection of cosmology and fundamental physics. If we can understand what it is, how it behaves, and where it came from, we can explain how the cosmos was built into the web of clusters and filaments we observe today. If we can identify dark matter as a new fundamental particle, we will have the first major departure from the standard model of particle physics—a paradigm that has held up to nearly every experimental test to an astounding degree of accuracy. Dark matter has the potential to open up the world of supersymmetry, a theoretical extension of the standard model that posits the existence of a whole new set of fundamental particles normally hidden from us. If the dark matter particle turns out not to be consistent with supersymmetry’s predictions, a huge branch of theoretical physics gets sent back to the drawing board.
So what do we know? We know dark matter is real—the evidence is overwhelming. Something must be responsible for the extra gravity messing with the motions of stars and galaxies. If that doesn’t convince you, you can look to gravitational lensing—the bending of light around massive objects. The presence of dark matter accounts for the way the light from distant stars and galaxies is distorted as it travels through the universe, following the gravitationally induced curving of space-time itself. If motion and lensing don’t convince you, look at the evidence that galaxies existed within a billion years of the Big Bang. Without dark matter as a kind of cosmic glue, galaxies would have taken much longer to form, as the gravity of gas and dust and stars had to fight against the pressure of all that matter colliding and heating up. Or just take a look at the Bullet Cluster. It’s the aftermath of a cosmic collision in which clusters of galaxies collided but the bulk of the matter passed right through the collision, in the way only ghostly dark matter could.
Even if somehow the clues have misled us completely, and some physical process were presented that mimicked dark matter without a new fundamental particle, it would require a rethinking of the basic laws of gravity and lead to a major revolution in our understanding of the universe.
What else do we know about dark matter? We know it doesn’t interact with electromagnetism in any significant way. It doesn’t absorb or emit light, and it doesn’t seem to experience friction or collisions with itself or other matter. It is probably passing through you right now, but without electromagnetic interactions, you won’t feel a thing. We know that it’s “cold,” in the sense that whatever makes it up is not moving at anything close to the speed of light. (Exactly how cold is still a matter of some debate—it could still be a little bit warm, or, perhaps, tepid.) We’re pretty sure dark matter is some kind of particle, but as far as we’ve seen, the only way dark matter particles interact with anything at all is through their gravity. Our theories (and our hopes!) depend on the possibility that dark matter can interact, if only rarely, via another kind of force: the weak force.
The idea of a weakly interacting massive particle, or WIMP, has been around for quite some time. The neutrino is a known example. Like the particle making up cosmic dark matter, the neutrino remains aloof to electromagnetic interactions, has mass, and interacts only via the weak force. This is the force that governs nuclear decay and fusion—most of the neutrinos we detect are byproducts of nuclear interactions in the sun. In a sense, neutrinos are dark matter, but they’re probably not the dark matter. The flavors of neutrinos we know about are too “hot” and not numerous enough to account for whatever it is that’s so ubiquitous in the cosmos. But attempts to detect dark matter rely on principles similar to those we used to discover neutrinos.
Dark matter searches come in three basic forms. The first, direct detection, is analogous to the search for neutrinos. Like neutrino detectors, dark matter detectors are built deep underground, where they are shielded from the constant bombardment of cosmic rays. We put our detectors in environments as protected as possible from radioactivity so that the detector’s particles are totally undisturbed. Then we wait. The expectation from leading dark matter theories is that, very rarely, one of the dark matter particles will bump into one of the detector’s particles, and if it’s a direct enough hit, it will recoil via the weak interaction. It’s not an easy experiment—the interaction is so rare that the number of direct hits is incredibly low, and even the best shielding can’t protect the instruments from every possible invading particle. There are a handful of detectors around the world, all watching carefully for a bump in the night, but so far, the results are uncertain. Some experiments seem to see signals, while others seem to rule them out. We don’t know yet whether the confusion is some strange form of dark matter that interacts differently with different materials, or whether a statistical fluke or the background noise of non-dark-matter particles has misled the experimenters.
Others are attempting to find dark matter via indirect detection. This relies on a weird possible quirk of dark matter particles. According to many theories, a dark matter particle is its own antiparticle, which means that if two meet each other, they annihilate completely into other particles or radiation. While the annihilation would be extremely rare, relying on an incredibly direct hit, it could produce standard-model particles that our telescopes and particle detectors could see. An experiment on the International Space Station called AMS recently found an excess of positrons (antiparticles of the electron) that they suspect could come from dark matter annihilation. But the jury is still out—it turns out there are a lot of things in the universe that create positrons.
If you can’t detect dark matter directly or indirectly, why not just to make it yourself? Experiments at CERN’s Large Hadron Collider rely on the idea that if two dark matter particles can annihilate to create two standard-model particles, it should be possible to reverse the reaction. If you slam protons together hard enough, you just might get dark matter particles. The LHC wouldn’t see those particles, of course, but it would see that energy in the interaction went missing, and that would be evidence of something really weird.
So far, the LHC has seen no evidence of dark matter. Direct and indirect detection experiments are inconclusive. The situation remains that the only evidence for dark matter’s existence is cosmological—the observations of the shape and motion of the cosmos—and that relies entirely on dark matter’s gravitational high jinks. Could it be that dark matter is truly undetectable? That it doesn’t interact with anything besides gravity at all? It’s possible, but theoretically that would be a disaster (or, alternatively, the stuff of scientific revolution). We rely on hypothetical interactions with standard-model particles to explain dark matter’s existence in the early universe, and if it has none, we have no way to explain it being here at all.
So, for the moment, the mystery remains. We are awash in clues, and confounded by conundrums. We continue to search for new dark matter phenomena and to find creative ways to uncover its secrets. Personally, I’m looking for evidence that it messed with the early formation of stars and galaxies. Some of my colleagues are studying the detailed structure of the dark matter surrounding nearby galaxies. Still others continue to improve our detectors and data analysis to see whether we can tease out a signal from the noise.
As we find more clues, we will keep trying to fit them into a single, coherent picture of the cosmos. The appeal of dark matter is that it seems to leave us hints everywhere we look, and its solution seems—almost—close enough to touch.
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