On September 23, 1846, Johann Gottfried Galle, Director of the Berlin Observatory, received a letter that would change the course of astronomical history. It came from a Frenchman, Urbain Le Verrier, who had been studying the motion of Uranus and concluded that its path could not be explained by the known gravitational forces acting on it. Le Verrier suggested the existence of a hitherto unobserved object whose gravitational pull was perturbing Uranus's orbit in precisely the way required to account for the anomalous observations. Following Le Verrier's directions, Galle went to his telescope that night and discovered the planet Neptune.

A similar drama—in which astronomers observe anomalous cosmic motions, deduce the presence of new matter and go out to hunt for it—is playing out again today in modern cosmology. In the role of Uranus, we see stars and galaxies moving in ways they should not; in the role of Neptune, we deduce the existence of hitherto unobserved substances, provisionally called dark matter and dark energy. From the types of anomalies we see, we can glean a few basic facts about them. Dark matter seems to be a sea of invisible particles that fills space unevenly; dark energy is spread out uniformly and acts as if it is woven into the fabric of space itself. Scientists have yet to repeat Galle's accomplishment of pointing an instrument at the sky and glimpsing the unseen players definitively, but tantalizing inklings, such as blips in particle detectors, continue to accumulate.

From its discovery as a shadowy force on Uranus, Neptune proved to be a fascinating world in its own right. Might the same be true of dark matter and dark energy? Scientists are increasingly considering the possibility that dark matter, in particular, is not just a contrivance to account for the motion of visible matter but a hidden side of the universe with a rich inner life. It may consist of a veritable zoo of particles interacting through novel forces of nature—an entire universe interwoven silently with our own.

The Dark Side
These ideas are a shift from the long-held assumption that dark matter and dark energy are the most antisocial substances in the cosmos. Since astronomers first inferred the existence of dark matter in the 1930s, they have considered inertness its defining property. Observations suggest it outweighs ordinary matter by a factor of 6 to 1. Galaxies and galaxy clusters are embedded in giant balls, or “halos,” of dark matter. For such a mass of material to elude direct detection, astronomers reason that it has to consist of particles that scarcely interact with ordinary matter or, indeed, with one another. All they do is provide the gravitational scaffolding for luminous matter.

Astronomers think the halos formed early on in cosmic history and then drew in ordinary matter, which, being capable of a rich range of behaviors, developed into intricate structures, while dark matter, being inert, remained in its primitive state. As for dark energy, its only role appears to be to accelerate cosmic expansion, and the available evidence indicates it has remained completely unchanged over the life of the cosmos.

The prospect that dark matter might be rather more interesting is driven not so much by the field of astronomy but by detailed investigations of the inner workings of atoms and the world of subatomic particles. Particle physicists have a tradition of seeing glimmers of unknown forms of matter in the behavior of known matter, and their evidence is completely independent of cosmic motions.

In the case of dark matter, the train of thought began with the discovery of radioactive beta decay in the early 1900s. Italian theorist Enrico Fermi sought to explain the phenomenon by postulating a new force of nature and new force-carrying particles that caused atomic nuclei to decay. This new force was similar to electromagnetism and the new particles to photons, the particles of light—but with a key twist. Unlike photons, which are massless and therefore highly mobile, Fermi argued that the new particles had to be heavy. Their mass would limit their range and account for why the force causes nuclei to fall apart but otherwise goes unnoticed. To reproduce the observed half-life of radioactive isotopes, they had to be quite heavy—around 100 times that of the proton, or about 100 giga-electron-volts, in the standard units of particle physics.

The new force is now known as the weak nuclear force, and the hypothesized force-carrying particles are the W and Z particles, which were discovered in the 1980s. They are not dark matter themselves, but their properties hint at dark matter. A priori, they should not be so heavy. Their high mass suggests that something is acting on them—novel particles that cause them to take on mass like a friend who encourages you to give into temptation and eat another slice of cake. One goal of the Large Hadron Collider is to look for those particles, which should have masses comparable to those of the W and Z. Indeed, physicists think dozens of types of particles may be waiting to be discovered—one for each of the known particles, paired off in an arrangement known as supersymmetry.

These hypothetical particles include some collectively known as weakly interacting massive particles, or WIMPs. The name arises because the particles interact only by means of the weak nuclear force. Being immune to the electric and magnetic forces that dominate the everyday world, they are totally invisible and have scarcely any direct effect on normal particles. Therefore, they make the perfect candidate for cosmic dark matter.

Whether they can truly explain dark matter, though, depends on how many of them there are. Here is where the particle physics argument really gains traction. Like any other breed of particle, WIMPs would have been produced in the fury of the big bang. High-energy particle collisions back then both created and destroyed WIMPs, allowing a certain number of them to exist at any given moment. This number varied with time depending on two competing effects driven by the expansion of the universe. The first was the cooling of the primordial soup, which reduced the amount of energy available to create WIMPs, so that their number diminished. The second effect was the dilution of particles, which reduced the frequency of collisions until they effectively ceased to occur. At that point, about 10 nanoseconds after the big bang, the number of WIMPs became frozen in. The universe no longer had either the energy needed to create WIMPs or the dense concentrations of mass needed to destroy them.

Given the expected mass of WIMPs and the strength of their interactions, which govern how often they annihilate one another, physicists can easily calculate how many WIMPs should be left over. Rather amazingly, the number matches the number required to account for cosmic dark matter today, within the precision of the mass and interaction-strength estimates. This remarkable agreement is known as the WIMP coincidence. Thus, particles motivated by a century-old puzzle in particle physics beautifully explain cosmological observations.

This line of evidence, too, indicates that WIMPs are inert. A quick calculation predicts that nearly one billion of these particles have passed through your body since you started reading this article, and unless you are extraordinarily lucky, none has had any discernible effect. Over the course of a year you might expect just one of the WIMPs to scatter off the atomic nuclei in your cells and deposit some meager amount of energy. To have any hope of detecting such events, physicists set their particle detectors to monitor large volumes of liquid or other material for long periods. Astronomers also look for bursts of radiation in the galaxy that mark the rare collision and annihilation of orbiting WIMPs. A third way to find WIMPs is to try to synthesize them in terrestrial experiments.

Out-Wimping the WIMPs
The extraordinary effort now being devoted to WIMP searches might leave the impression that these particles are the only theoretically plausible dark matter candidate. Are they? In fact, recent developments in particle physics have uncovered other possibilities. This work hints that the WIMP is just the tip of the iceberg. Lurking under the surface could be hidden worlds, complete with their own matter particles and forces.

One such development is the concept of particles even more wimpy than WIMPs. Theory suggests that WIMPs formed in the first nanosecond of cosmic history might have been unstable. Seconds to days later they could have decayed to particles that have a comparable mass but do not interact by the weak nuclear force; gravity is their only connection to the rest of the natural world. Physicists, tongue in cheek, call them super-WIMPs.

The idea is that these particles, rather than WIMPs, constitute the dark matter of today's universe. Super-WIMPs would elude direct observational searches but might be inferred from the telltale imprint they would leave on the shapes of galaxies. When created, super-WIMPs would have been moving at a significant fraction of the speed of light. They would have taken time to come to rest, and galaxies could not have begun forming until they did. This delay would have left less time for matter to accrete onto the centers of galaxies before cosmic expansion diluted it. The density at the center of dark matter halos should therefore reveal whether they are made of WIMPs or super-WIMPs; astronomers are now checking. In addition, the decay from WIMP to super-WIMP should have produced photons or electrons as a by-product, and these particles can smash into light nuclei and break them apart. There is some evidence that the universe has less lithium than expected, and the super-WIMP hypothesis is one way to explain the discrepancy.

The super-WIMP scenario also inspires fresh possibilities for what experimental physicists might observe. For instance, the original WIMP need not have been either dark or wimpy; it could have had an electric charge. Any charge it had would not have affected the evolution of the cosmos, because the particle decayed so quickly. It would, however, mean that WIMPs would be extremely conspicuous if experimentalists were able to re-create them. Particle detectors would register them as electrons on steroids; having the same charge as an electron but 100,000 times more mass, such a particle would barrel through the detectors, leaving spectacular tracks in its path.

Dark Forces, Hidden Worlds
The main lesson of super-WIMP models is that there is no reason, either theoretically or observationally, that dark matter should be as boring as astronomers tend to presume. Once one admits the possibility of hidden particles with properties that go beyond the standard WIMP scenario, it is natural to consider the full range of possibilities. Could there be a whole sector of hidden particles? Could there be a hidden world that is an exact copy of ours, containing hidden versions of electrons and protons, which combine to form hidden atoms and molecules, which combine to form hidden planets, hidden stars and even hidden people?

The possibility that a hidden world could be identical to ours has been explored at length, beginning in 1956 with an offhand comment in a Nobel Prize–winning paper by Tsung-Dao Lee and Chen Ning Yang and more recently by many others, including Robert Foot and Raymond Volkas of the University of Melbourne in Australia. The idea is truly tantalizing. Could it be that what we see as dark matter is really evidence for a hidden world that mirrors ours? And are hidden physicists and astronomers even now peering through their telescopes and wondering what their dark matter is, when in fact their dark matter is us?

Unfortunately, basic observations indicate that hidden worlds cannot be an exact copy of our visible world. For one, dark matter is six times more abundant than normal matter. For another, if dark matter behaved like ordinary matter, halos would have flattened out to form disks like that of the Milky Way—with dramatic gravitational consequences that have not been seen. Last, the existence of hidden particles identical to ours would have affected cosmic expansion, altering the synthesis of hydrogen and helium in the early universe; compositional measurements rule that out. These considerations argue strongly against hidden people.

That said, the dark world might indeed be a complicated web of particles and forces. Several investigators, including one of us (Feng) and Jason Kumar of the University of Hawaii at Manoa, have found that the same supersymmetric framework that leads to WIMPs allows for alternative scenarios that lack WIMPs but have multiple other types of particles. What is more, in many of these WIMP-less theories, these particles interact with one another through newly postulated dark forces. We found that such forces would alter the rate of particle creation and annihilation in the early universe, but again the numbers work out so that the right number of particles are left over to account for dark matter. These models predict that dark matter may be accompanied by a hidden weak force or, even more remarkably, a hidden version of electromagnetism, implying that dark matter may emit and reflect hidden light.

This “light” is invisible to us, and so the dark matter remains unseen. Still, new forces could have very significant effects. For example, they could distort interacting clouds of dark particles. Astronomers have sought this effect in the famous Bullet cluster, which consists of two clusters of galaxies that have passed through each other. Observations show the co-mingling of clusters left the dark matter largely unperturbed, indicating that any dark forces are weak. Researchers are continuing to look in other systems.

Such forces would also let dark particles exchange energy and momentum with one another, a process that would tend to homogenize them and cause once lopsided halos to become spherical. This process should be most pronounced for dwarf galaxies, where the dark matter is slow-moving, particles linger near one another and small effects have time to build up. The observation that small galaxies are systematically rounder than their larger cousins would suggest dark matter interacting through new forces. Astronomers are only just beginning these studies.

From One Dark Thing to Another
An equally intriguing possibility is that dark matter interacts with dark energy. Most theories treat the two as disconnected, but there is no reason they must be, and physicists are now considering how the two might affect each other. One hope is that couplings between them might mitigate some cosmological problems, such as the coincidence problem—the question of why the two have comparable densities. Dark energy is roughly three times as dense as dark matter, but the ratio might have been far larger. This coincidence would make sense if dark matter somehow triggered the emergence of dark energy.

Couplings with dark energy might also allow dark matter particles to interact in ways that ordinary particles do not. Recent models allow and sometimes even mandate dark energy to exert a different force on dark matter than it does on ordinary matter. Under the influence of this force, dark matter would tend to pull apart from any ordinary matter it had been interlaced with. In 2006 Marc Kamionkowski, then at the California Institute of Technology, and Michael Kesden, then at the Canadian Institute for Theoretical Astrophysics, suggested looking for this effect in dwarf galaxies being torn apart by their larger neighbors. The Sagittarius dwarf galaxy, for example, is being eaten by the Milky Way, and astronomers think its dark matter and ordinary matter are spilling into our galaxy. They calculate that if the forces acting on dark matter are at least 4 percent stronger or weaker than the forces acting on ordinary matter, then the two components should drift apart by an observable amount. At present, however, the data show nothing of the sort.

Another idea is that interacting dark matter and dark energy would alter the growth of cosmic structures, which depends delicately on the composition of the universe. A number of researchers, including one of us (Trodden), with collaborators Rachel Bean, Éanna Flanagan and Istvan Laszlo of Cornell University, have used this constraint to rule out a large class of models.

Despite these null results, the theoretical case for a complex dark world is now so compelling that many researchers would find it more surprising if dark matter turned out to be nothing more than an undifferentiated swarm of WIMPs. After all, visible matter comprises a rich spectrum of particles with multiple interactions determined by beautiful underlying symmetry principles, and nothing suggests that dark matter and dark energy should be any different. We may not encounter dark stars, planets or people, but just as we could hardly imagine the solar system without Neptune, Pluto and the swarm of objects that lie even farther out, one day we might not be able to conceive of a universe without an intricate and fascinating dark world.