Dark matter is as tangible as stars and planets to most astronomers. We routinely map it out. We conceive of galaxies as lumps of dark matter with dabs of luminous material. We understand the formation of cosmic structure, as well as the evolution of the universe as a whole, in terms of dark matter. Yet a decade of sophisticated searches has failed to detect the material directly. We see the shadow it casts, but are completely unaware of what the dark side of the universe may contain.
It certainly isn’t any ordinary object or particle—that has long since been ruled out. Theoretical prejudice favors a novel type of particle that interacts only weakly with ordinary matter. Vast numbers of these particles should be flowing through our planet all the time, and by rights you’d expect some of them to leave a mark. Physicists have grown crystals and filled cryogenic vats, hauled them deep underground to screen out run-of-the-mill particles, and watched for tiny pulses of heat and flashes of light that would betray the passage of something never before seen. The results so far are not encouraging. In Lead, South Dakota, the LUX experiment operates one mile underground in an abandoned gold mine. It has found nothing. In China, the PandaX experiment in the Jin-Ping underground laboratory operates in a tunnel under 2.4 kilometers of rock. It has found nothing. In a road tunnel near Fréjus in the French Alps, the EDELWEISS experiment, at a depth of 1.7 km, has found nothing. And the list goes on.
The null results are rapidly squeezing the regions of parameter space where dark matter might lurk. Confronted by the drought of data, theoretical physicists have conjectured about more exotic particles, but the vast majority of these candidates would be even harder to detect. One could instead hope to produce dark-matter particles at a particle accelerator, so that we could infer their presence by default: by checking whether energy seemed to go missing in particle collisions. But the Large Hadron Collider has tried precisely this and noticed nothing so far. Some theorists suspect dark matter doesn’t exist and our theory of gravitation—Einstein’s general theory of relativity—has led us astray. General relativity tells us that galaxies would fly apart if not held together by unseen matter, but perhaps the theory is wrong. Yet general relativity has passed all other observational tests, and all rival theories have seemingly fatal flaws.
Eighty-five percent of all matter is unknown. Our greatest fear is that it will always remain so.
Although most experiments have come up short, two do claim to have spotted dark matter. Both claims are highly controversial, for different reasons. These outliers may well be wrong, but they deserve a closer look. If nothing else, these cases illustrate the difficulty of spotting dark matter amid all the other detritus of the cosmos.
The DAMA/LIBRA particle detector at the Gran Sasso Laboratory, installed in a tunnel 1.4 kilometers below a mountain in northern Italy, looks for flashes of light caused by dark-matter particles scattering off the atomic nuclei in a crystal of sodium iodide. It has been collecting data for over 13 years and has seen a very peculiar thing. The rate of particle detections waxes and wanes with the seasons, with a maximum in June and a minimum in December.
We see the shadow it casts, but are unaware what the dark side of the universe may contain.
That is exactly what you expect from dark matter. Dark matter is thought to form a vast cloud enveloping the Milky Way galaxy. Our solar system as a unit is moving through this cloud. But individual planets move through the cloud at a varying speed because of their orbital motion around the sun. Earth’s speed relative to the putative cloud peaks in June and bottoms out in December. That would determine the rate at which dark-matter particles flow through an Earth-based detector.
There is no denying that DAMA detects such a seasonal modulation with a very high statistical significance. But many other sources of particles also vary with the seasons, such as groundwater flows (which affect the background level of radioactivity) and the production of other particles, such as muons, in the atmosphere. At last count, some five other experiments around the world claim limits that are inconsistent with DAMA’s claim. But the only way to be sure is to replicate the experiment with the same type of detector at one or more different locations, and several such experiments are now underway. One will be at the South Pole, where local seasonal effects are out of phase and very different from those in Italy.
A second intriguing hint of dark matter comes from indirect experiments, which do not look for the elusive particles per se, but for the secondary particles they would produce when they collide with one another and mutually annihilate. In 2008 an Italian-Russian satellite called PAMELA (Payload for Antimatter/Matter Exploration and Light-nuclei Astrophysics) observed an unexpectedly high number of positrons, the antimatter version of the electron, emanating from deep space. The observation was recently confirmed by the Alpha Magnetic Spectrometer onboard the International Space Station. Meanwhile, the Fermi satellite has reported a diffuse glow of gamma rays extending up to around 20 degrees from the center of our galaxy. It has just the shape expected from dark matter: spherically symmetric about the galactic center, with an intensity that rises toward the middle.
It almost seems too good to be true. Unfortunately, both the positron and gamma-ray observations might be explained by rapidly rotating neutron stars known as millisecond pulsars. The positrons just don’t match the signature of viable dark-matter candidates. To settle the case, we need to check whether the positrons tend to be coming from the direction of known neutron stars. Fluctuations in the gamma rays already tend to favor the option of many weak and unresolved pulsar sources near the galactic center. Also, if the gammas were coming from dark matter, astronomers should detect a similar signal from nearby small dwarf galaxies, which have a proportionately greater amount of dark matter than our own galaxy. No such signal has been detected.
Most of our search efforts have focused on the simplest candidate particles, known as WIMPs: weakly interacting massive particles. The word “weakly” is a double entendre: The interaction is feeble, and it occurs via the so-called weak nuclear force. Such particles are a natural extension of the Standard Model of particle physics. Even without knowing the details, the adverb “weakly” is enough information to calculate how many such particles should suffuse the universe. In the hot primordial soup of the big bang, particles are naturally created and destroyed. As the universe expands, the temperature drops, and one by one, different types of particles cease to form, depending on their mass. Particles can still be destroyed at a rate that depends on their interaction strength, until they are too diffusely spread out to collide with one another.
Given the interaction strength that WIMPs should have, you can run the numbers, and you find that the cauldron of the early universe should have created the observed amount of dark matter. The resulting particles should weigh in at hundreds of proton masses. In sum, there is a natural sweet spot for particle dark matter, dubbed the “WIMP miracle.”
But maybe this is a case of a beautiful hypothesis slain by an ugly fact. Physicists are becoming increasingly desperate, exploring options that they used to consider distant second-best possibilities.
We are in a situation scientists dream of. Old ideas aren’t working; new ones are needed.
Perhaps the dark-matter particles are exceedingly massive. There is a basic tradeoff, though. The more massive the particle is, the fewer of them are needed to account for the total mass that astronomers observe, and there might be so few that our detectors would miss them. Physicists would need to find some completely alternative search strategy, perhaps involving the effects these particles might have on old neutron stars or other celestial objects.
Going the other direction, the dark-matter particle might be too lightweight to leave much of a mark in our detectors. To search for it, physicists might make use of a detector that nature has already provided for us: the sun. The sun might sweep up particles as it moves through the galactic dark-matter cloud. These particles could scatter off protons in the sun and modify its temperature profile. That would affect the turbulent motions of gas eddies that rise, fall, and swirl in the sun’s upper layers. And we should be able to see that through the science of helioseismology, which studies disturbances that propagate inside the sun and their effect on the surface, much as we study terrestrial earthquakes by seismology. It turns out that there are unexplained helioseismological anomalies that are difficult to reconcile with our standard model of the sun.
If dark-matter particles collect in the sun, they may also annihilate in the core. That would produce energetic neutrinos that detectors such as Super-Kamiokande in central Japan and the IceCube observatory at the South Pole could see. So far no candidate events have been reported.
The most extreme example of an ultra-lightweight particle is the axion, a hypothesized weakly interacting particle with a trillionth or less of the mass of a proton. It would not be completely dark, but would interact electromagnetically and could generate microwave photons inside strong magnetic field cavities. Experiments aimed at detecting axions have been operating since the 1980s, with as little success as WIMP detectors.
Perhaps the dark particle is not even a particle, but an “unparticle,” as dubbed by one theorist. Unparticles are distant cousins of the electromagnetic field whose energy does not come in discrete packages. They could leave an indirect trace in collider data. Perhaps the identity of dark matter has no single solution. After all, ordinary matter is also composed of many types of particles. Dark matter could likewise have several contributors, rendering the search more difficult by diluting the putative signature of any specific particle candidate. Perhaps dark matter does not interact at all, except gravitationally. This would make the experimentalist’s life even more of a nightmare.
In a sense, we are in the situation every scientist dreams of. Old ideas aren’t working; new ones are needed. These might come from exploring novel types of particles, or we might discover a fully consistent new theory of gravity that dispenses entirely with dark matter.
The nagging worry is that nature has put the new physics in a place where we can’t find it. Although we haven’t completely exhausted the search for WIMPs, there’s only so much more that experiments can do. As they become more sensitive to dark matter, they also become more sensitive to garbage particles, and they cannot always discriminate between the two. At the present rate of improvement, within a decade they will be blinded by neutrinos emitted either by the sun or by cosmic rays colliding with Earth’s atmosphere.
At that point, we could still pursue indirect means of detection. One of the most promising is the Cherenkov Telescope Array, an assembly of more than 100 telescopes in Chile and on La Palma, which, among other goals, will look for gamma rays produced by annihilation of dark-matter particles in our galaxy and others. But eventually this search strategy will run into another problem: cost. For now, dark-matter detectors are among the most economical of major physics experiments, but if we need to keep increasing their size, sensitivity, and sophistication, their price tag could rival behemoths such as the Large Hadron Collider (nearly $7 billion to build) and the James Webb Space Telescope (around $8 billion), with no guarantee of success—a very hard sell for politicians.
The strongest tool for discovery of dark-matter particles would be a new particle collider. Fast-forwarding some three decades from now, physicists plan to build a collider with seven times the power of the LHC. Studies are underway both in China and in Europe. Crudely scaling up from the LHC, it would cost $25 billion in today’s dollars. Shared among nations and spread over the decades, that might just be feasible. But it is probably the limit. Even if physicists had unlimited resources, nothing would be gained by building anything larger. At that point, any unknown particle would have to be so massive that, were the particle produced in the same way as its lighter counterparts, the big bang would not have produced it in sufficient quantity.
Despite these immense efforts, we may not find any signals. That would be a gloomy prospect. Maybe there is no dark matter. We keep looking for deviations from general relativity. So far we have found none. On the contrary, the detection of black holes in 2016 by gravitational waves has bolstered Einstein’s theory—and its corollary, the existence of dark matter.
But look on the bright side. There could be immense mysteries and revelations about the dark side of nature that we will never glimpse unless we search. For now, we keep looking for particles. We can do nothing else but press on.