The beautiful spinning pinwheel of the Andromeda galaxy, our celestial neighbor, poses a mystery. The breakneck speed of its rotation cannot be explained by applying the known laws of physics to the disk's visible matter. By rights, the gravity generated by the galaxy's apparent mass should cause the stars in the periphery to move more slowly than they actually do. If the visible matter was all there was, Andromeda, and nearly all such quickly rotating galaxies, simply should not exist.

Cosmologists believe that some unseen kind of matter—dark matter—surrounds and permeates Andromeda and other galaxies, adding the necessary gravitational force to keep them spinning as observed. Dark matter, which appears to contribute about 25 percent of the universe's mass, would also explain other aspects of the cosmos, including the exceedingly fast motion of galaxies within clusters of galaxies, the distribution of matter arising when two clusters collide and the observation of gravitational lensing—the bending of light by gravity—of distant galaxies.

The simplest theories of dark matter postulate a single kind of as yet undiscovered particle contributing the unseen mass. But despite decades of searching for direct evidence of the dark matter particle, no one has been able to prove its existence. Further, a few discrepancies remain between astronomical observations and this simple theory. The combination of these residual disagreements with the failure to detect this elusive substance has led some scientists to question the traditional theories and imagine a more complicated form of dark matter. Instead of a single type of particle, dark matter might be made of a wider array of dark species. After all, ordinary matter comes in many forms—maybe dark matter is similarly complex.

Over the past few years scientists have increasingly come to suspect that several varieties of dark matter exist and, perhaps even more intriguing, that previously unsuspected forces act strongly on dark matter and very feebly (or not at all) on ordinary matter. Recent observations of colliding galaxies may provide preliminary support for this hypothesis, and such forces could help explain some of the discrepancies between the basic dark matter model and observations. If complex dark matter exists, it would make for a more interesting and intricate universe than cosmologists usually imagine.

Hidden matter

Although we do not yet know what constitutes dark matter, we do know something about its properties from our observations of how it influences normal matter and from simulations of its gravitational effects. For instance, it must be moving much slower than the speed of light; otherwise the density fluctuations present in the early universe would not lead to the galactic structures observed today. Because it does not absorb or emit electromagnetic radiation, it must be electrically neutral. The particles that compose dark matter are probably massive, or else they would have to be moving near the speed of light, which data from the early universe rule out. They cannot interact via the strong force, which binds atomic nuclei together; otherwise we would have seen evidence in dark matter's interaction with high-energy charged particles called cosmic rays. Until recently, scientists believed that dark matter might interact via the weak force (responsible for radioactive decay), but new observations have undercut that notion. (Although it remains possible that dark matter could still experience weak force interactions, to be consistent with observations such interaction is plausible only if additional as yet undetected particles exist besides dark matter.)

We also know that dark matter must be stable on cosmic timescales. The reason is simple: there is no credible mechanism to continually produce dark matter; thus, dark matter must be primordial, meaning that it originated in the big bang. Saying a particle is stable hides a profound truth; its stability tells us that it possesses a property that is “conserved”—it cannot change—and thereby forbids the particle to decay, which would alter the conserved property. We can illustrate the meaning of this term by invoking the familiar electrical charge, which ensures that the electron is stable. It is a truism of physics that particles decay into lighter ones unless something prevents that decay. The electron is electrically charged, and the only known stable particles lighter than it are electrically neutral: the photon and the neutrinos. Energy considerations would allow the electron to decay into these objects, but because conservation of charge prohibits such decays, the electron stays an electron.

Most dark matter theories assume the dark particles have a conserved quantity called, for historical reasons, parity, with the dark matter particle having a parity of −1 and all other known particles having a parity of +1. A dark matter particle is then forbidden from decaying into ordinary matter by this parity because if the dark entity disappeared and ordinary particles appeared, the parity would not be conserved.

The simplest theory that meets all the conditions physicists have outlined posits a single particle responsible for dark matter called a WIMP, for weakly interacting massive particle. (The term “weakly” here is used in the generic sense and does not necessarily mean the weak nuclear force.) WIMPs make sense for many theoretical reasons, but they are proving harder to find than many physicists expected. Since the 1990s scientists have been running various experiments aimed at directly detecting WIMPs through their very rare interactions with ordinary matter.

To achieve the necessary sensitivity, the detectors are cooled to extremely low temperatures and buried deep underground to shield them from ubiquitous cosmic rays, which can mimic a dark matter signature. Yet despite increasingly powerful experiments, no conclusive sign of WIMPs has emerged. And whereas the WIMP model does explain many aspects of our observed universe, it does not account for everything. For example, WIMP theories predict that a much greater number of small satellite galaxies should be orbiting the Milky Way than apparently do swirl around it and that dark matter should be even denser in the center of galaxies than it seems to be based on the galaxies' observed rotation rates. The situation is evolving rapidly, however—the recent discovery of additional satellite galaxies by the Dark Energy Survey collaboration suggests the problem with the Milky Way's dwarf galaxies may simply be that many have yet to be found.

Ultimately, though, these WIMP shortcomings have left the door open for more unconventional dark matter models.

Complex dark matter

Rather than a single particle constituting all of dark matter, one could imagine that several classes of dark matter particles exist, as well as a variety of forces that act only on dark matter. One idea that appears to reconcile all the observations and simulations is the possibility that dark matter particles interact with one another—essentially, dark matter particles may feel a force between them that is not felt by ordinary matter. These particles could, for instance, carry a new kind of “dark charge” that attracts or repels them while leaving them electrically neutral. Just as ordinary particles with electrical charge can emit photons (particles of light that are the carriers of the electromagnetic force), perhaps particles with dark charge could emit “dark photons”—not particles of light but rather particles that interact with dark charge in the same way that photons interact with electrical charge.

The parallels to the world of normal matter must end at a certain point, however. The reason we know that is the following: Suppose that the rules of the dark world exactly mirrored ours. In that world, dark atoms would form and emit dark photons at the same rate that ordinary matter emits ordinary photons. In our world, the emission of photons allows energy to be exchanged and is the reason galaxies eventually relax into disklike objects. Clouds of gas inside galaxies radiate electromagnetic energy, which results in the matter inside the clouds clumping together. Conservation of angular momentum precludes matter from contracting to a point, but a disklike structure forms easily. If the rules and forces governing the behavior of dark matter were the same as ours, the emission of dark photons would result in all dark matter galaxies forming flattened disks. Yet we know that the distribution of most of the dark matter required to explain our familiar galaxies is more like a spherical cloud. Thus, we can rule out an exact mirror world of dark matter.

Still, many alternatives remain. For instance, it is possible that a small fraction of dark matter mirrors the rules of our universe, whereas the larger fraction acts more like the simple WIMPs. Or perhaps the dark charge is effectively much smaller than the electrical charge of our electrons and protons, resulting in reduced dark photon emission. Theorists, including one of us (Dobrescu), are generating many ideas about possible particles and forces of the dark sector, using existing data to guide our thinking and constrain speculations. One of the simplest scenarios—involving just two kinds of dark matter particles—offers a glimpse of some of the physics that could operate in complex dark matter.

Dark photons

Imagine a dark world in which two kinds of dark charge exist—one positive and one negative. In this model, there is a form of dark electromagnetism, leading the dark matter particles to emit and absorb dark photons. Because, as postulated, these particles are charged in a way analogous to ordinary electromagnetism, positively and negatively charged dark matter particles should be able to meet and annihilate into dark photons, just as normal matter particles and their oppositely charged antimatter counterparts annihilate on contact, releasing photons.

We can make some conclusions about the strength of the dark electromagnetism force and thus how often dark matter annihilation occurs by considering how this force would affect galaxies. Recall that the reason galaxies have a flattened structure is that electromagnetism allows ordinary matter to lose energy and settle into disks. This energy loss occurs even without annihilation. Because we know that dark matter is primarily distributed spherically around most galaxies and does not collapse to a disk, we can conclude that it cannot lose energy via dark photon emission at the same rate that ordinary matter does. In a study published in 2009 Lotty Ackerman, Matthew R. Buckley, Sean M. Carroll and Marc Kamionkowski, all then at the California Institute of Technology, showed that this requirement implies that the dark charge must be very small, about 1 percent the value of the electrical charge. Yet even at such a low value, the force could still exist and effect significant consequences on galaxies.

Dark galaxy

So far we have described a version of dark matter consisting of a charged dark particle and its oppositely charged match emitting dark photons. But this scenario still pales in comparison to the complexity of ordinary matter. What would a dark matter world with multiple different charged particles look like?

There are many theories of complex dark matter that include two or more hypothetical dark particles. One particularly intriguing example was proposed in 2013 by JiJi Fan, Andrey Katz, Lisa Randall and Matthew Reece, all then at Harvard University, who referred to their model as “partially interacting dark matter.” They assumed the bulk of dark matter was made up of WIMPs but also postulated a small component consisting of two classes of particles known as fermions: one heavy and one light, both of which carry dark charge. (Fermions are particles with a quantum-mechanical spin of ½; in our familiar world, protons, neutrons and the quarks that compose them are examples of fermions.) Because the dark fermions carry dark charge, they emit dark photons and can be attracted to one another.

Although one must be very cautious to not overinterpret the correspondence, the proposed situation is broadly similar to postulating a dark proton, a dark electron and a dark photon to carry the dark electromagnetism that binds them together. Depending on the mass and charges of the dark fermions, they could combine to create dark atoms with their own dark chemistry, dark molecules and possibly even more complex structures. The concept of dark atoms was explored in detail in 2010 by David E. Kaplan, Gordan Z. Krnjaic, Keith R. Rehermann and Christopher M. Wells, all then at Johns Hopkins University.

The Harvard physicists who proposed the dark matter fermions idea went on to derive an upper limit on the fraction of dark matter that may be strongly interacting with dark photons, given the constraints imposed by astronomical observations. They determined that its cumulative mass may be as large as that of all visible matter. In this model, the Milky Way galaxy consists of a large spherical cloud of WIMP-like particles, which contributes 70 percent of the total matter, encircling two flattened disks, each containing 15 percent of the matter. One disk is normal matter, which includes the spiral arms that we can see, and the other consists of strongly interacting dark matter. The two disks need not be exactly aligned, but they would have a similar orientation. In this picture, a dark matter galaxy basically coexists in the same space as our familiar Milky Way. A cautionary note: the dark matter galaxy would not include dark stars or large planets, because these would have been observed through their gravitational-lensing effects on ordinary matter.

The idea may sound radical, but the extra disk in our galaxy would do little to change the normal matter cosmos with which it coexists. After all, to be correct, any theories about dark matter have to be consistent with existing observations of visible matter. We could be living in such a universe without even knowing it.

Experimental prospects

Scientists can search for complex dark matter in the same ways they search for WIMPs: with sensitive underground detectors. One consequence of the partially interacting dark matter model, with its concentrated disk of matter roughly in the same plane as the visible matter of the Milky Way, is that this form of dark matter passing through our detectors would be denser than that predicted in WIMP models. The increased density could result in a greater probability of these detectors finding dark matter than conventional theory predicts.

In addition to conducting such experiments, physicists hope to make dark matter in particle accelerators, along with all the other exotic particles generated there. Because we know very little about how dark matter interacts with ordinary matter—and thus which particular processes inside the accelerator might give rise to it—scientists have embarked on a broad program of investigation. This program is sensitive to a variety of models of dark matter, ranging from the simple WIMP to a more complex dark sector, although we must make some assumptions, such as that dark matter interacts with ordinary matter via a force or forces that are much stronger than gravity (the weakest of all known forces) yet weak enough to not yet have been observed. This assumption is necessary because if dark matter interacts only gravitationally, we will never create it in any conceivable accelerator, nor will we see it in any direct search. This force would be different from the chargelike force through which dark matter might interact with itself.

The Large Hadron Collider (LHC) at CERN near Geneva is the world's highest-energy accelerator, which gives it an edge when searching for heavier versions of dark matter (the more massive a particle is, the more energy it takes to produce it inside an accelerator), as well as for dark matter particles whose interactions become increasingly frequent as their energy rises. Because we already know dark matter can interact only very weakly with ordinary matter, we cannot expect to observe it directly in the detector, which is made of ordinary matter. Instead scientists search for dark matter by looking for collisions in which energy is missing. For example, two protons might collide and produce some ordinary particle or particles exiting one side of the collision and a couple of dark matter particles on the other. The signature of such an event is observed energy on one side of the detector with nothing on the other side. Scientists calculate how many collisions would be expected to show this striking configuration if dark matter did not exist and then look to see if there are more than expected.

So far no signs of such an excess have shown up inside the LHC—an indication that dark matter's interactions with ordinary matter must be very infrequent, if they occur at all. But a new opportunity for seeing signs of dark matter recently began with the start of the LHC's upgraded, higher-energy second run this spring. That means that the discovery of the century could be right around the corner.

In addition to the searches for dark matter we have just described, which are suitable for finding both WIMPs and complex dark matter, some approaches aim more specifically at the complex dark sector. Many of these search for the dark photon. Some models suggest that dark photons can continually transform into ordinary photons and back again via the laws of quantum mechanics, potentially presenting an opportunity to see the photons that result. Other models suggest certain dark photons have a nonzero mass (the use of the word “photon” is stretched in that case, in that they differ from the familiar massless photon). If a dark photon has mass, it can potentially decay into lighter particles. And because this dark photon can transform briefly into a normal photon, there is a small chance that it can produce pairs of electrons and their antimatter counterparts or similarly a matter-antimatter pair of muons (cousins of electrons) during the transformation process.

Consequently, experimental collaborations, including a project for which one of us (Lincoln) is a member, search for collisions that produce an electron-positron or a muon-antimuon pair. Such studies are ongoing at the LHC and at other accelerator facilities, such as at the KLOE-2 project at the National Institute for Nuclear Physics' Frascati National Laboratories in Italy, the Heavy Photon Search (HPS) experiment at the Thomas Jefferson National Accelerator Facility in Newport News, Va., and the BaBar detector experiment at SLAC National Accelerator Laboratory—and scientists are even digging through data more than a decade old taken by a SLAC experiment known as mQ.

Another interesting approach utilizes Fermi National Accelerator Laboratory in Batavia, Ill., to try to make beams of dark matter particles. Fermilab is currently generating intense beams of neutrinos that shoot at distant detectors. Neutrinos are very light subatomic particles that interact essentially exclusively through the weak nuclear force. If dark matter interacts with ordinary matter via particles like dark photons, it is possible that dark matter is being made in the same beams and can possibly be detected in Fermilab's MiniBooNE, MINOS or NOvA detectors.

Finally, scientists can search for astronomical signs that dark matter is interacting in situations such as when galaxies collide. In such scenarios, when the dark matter from one galaxy slams into the dark matter in another, the particles could repel one another by exchanging dark photons. Several studies of galaxy crashes have failed to find evidence of this phenomenon, but observations, published just a few months ago, of the cluster Abell 3827, which is particularly close to Earth and well oriented, hint at just such a pattern. Further observations of that and other galaxy collisions will be necessary to confirm the signal, but the data from this cluster so far look promising for complex dark matter models.

A cosmic stumper

There is no question that we are facing a profound conundrum. On large scales, ordinary gravitationally bound matter does not act in ways consistent with the known laws of physics and the observed distribution of mass. Because of this disagreement, most scientists are confident that some form of dark matter exists. What form this matter takes, however, has become increasingly contentious as our experiments repeatedly fail to find evidence for the simplest dark matter models. For this reason and because of some persistent discrepancies between the simple WIMP model predictions and astronomical observations, complex dark matter theories are becoming more appealing. These models offer theorists more parameters to tune and thus to improve the agreement between data and theory. They also more closely match the variation and richness of normal matter.

A criticism of this approach may be that it works overly hard to keep the dark matter hypothesis alive. Could this situation be similar to the discredited idea of epicycles, whereby 16th-century astronomers tried to retain geocentrism by adding a constant series of tweaks to a fatally flawed theory? We think not, given that dark matter explains many astronomical conundrums remarkably well and there is no a priori reason why dark matter should be as simple as the WIMP hypothesis.

The real message is that we have a mystery before us and that we do not know what the answer will be. Until we find it, we must be open to myriad explanations, including the fascinating possibility that we might be living alongside a dark parallel reality. Could it be that a dark matter scientist has turned its attention to its skies and is wondering about us?