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.