In the Dark About Dark Matter

Physicists have once again missed a long-planned meeting with their future. The latest and most important task for scientists is to search for a particle that is a component of dark matter—the invisible substance that makes up about 85% of the mass of the cosmos. These elusive particles, called WIMPs (weakly interacting massive particles), are likely simply better hidden than physicists think. Furthermore, assuming they don’t exist, scientists would have to rethink the entire structure of the universe. Many researchers still hope to find the elusive particles in updated versions of experiments, but others take a different view of the concept of dark matter, having long considered its existence unlikely.

The first disappointing result was obtained in the summer of 2017 by the LUX (Large Underground Xenon) experiment. A titanium tank containing a third of a ton of liquid xenon at -100 degrees Celsius was placed inside a giant water tank buried a mile beneath the Black Hills (a former mine in South Dakota). There, shielded from most sources of radiation contamination, researchers spent years looking for flashes of light emitted by other particles interacting with xenon atoms. But no WIMP particles were found, as announced on July 21st.

A second disappointing report was received on August 5th, 2017, from the world’s largest particle accelerator, the Large Hadron Collider (LHC). Since the spring of 2015, the LHC has been pursuing WIMPs, smashing protons together at unprecedented energies, at rates up to a billion collisions per second, pushing particle physics to new frontiers. Early on, two teams detected clear anomalies in the subatomic debris: excess energy in proton collisions, hinting at «new physics,» could have been produced by WIMPs (or the researchers had discovered a new particle in molecular physics). But after analyzing the data, the anomaly petered out, indicating it was a statistical fluke.

These two null results are like a double-edged sword for dark matter. On the one hand, their new mass limits (there are suggestions that WIMPs are massless) and WIMP interactions lay new groundwork for the next generation of detectors that could be successful. On the other hand, they rule out some of the simplest and most cherished WIMP models, raising new concerns that scientists may still have several decades to spend searching for dark matter.

Edward «Rocky» Kolb, now a cosmologist at the University of Chicago, helped lay the groundwork for the WIMP hunt back in the 1970s. He calls the 2010s «the Decade of the WIMP,» but admits that the search isn’t progressing as planned. «We’re in a much darker place now about dark matter than we were five years ago,» Kolb concludes. The scientist also notes that most theorists have reacted to this as «letting a thousand WIMPs bloom,» creating ever more baroque and exotic theories to explain how supposedly ubiquitous particles evade all our detectors.

Theorists have two interrelated reasons for hunting WIMPs. First, WIMPs are a natural consequence of the most popular extensions of the Standard Model of particle physics, which predicts their creation shortly after the Big Bang. Second, if such primordial WIMPs exist, simple calculations show that their abundance and behavior should now almost exactly match the quantities and properties of dark matter implied by observations. This so-called «wonder WIMP» has been sought for decades, but some theorists are now questioning its existence.

For example, in 2008, Jonathan Feng and Jason Kumar, then working at the University of California, Irvine, demonstrated how the well-known phenomenon of supersymmetry can produce a hypothetical class of particles much lighter and more weakly interacting than WIMPs. «These particles lead to the same amounts of dark matter we see today, but they’re not WIMPs,» says Feng. «This upsets the apple cart because it’s just as well-motivated theoretically. We call it a miracle, but less so than WIMPs.»

The dwindling theoretical support for simple WIMP models, coupled with a growing list of failed detection efforts, has led F eng and many others to propose that WIMPs are part of a more complex picture: a hidden realm of the universe filled with a variety of dark particles interacting with each other through a set of dark forces, possibly exchanging dark charges through bursts of dark light. Therefore, theorists should play with more variables, such as «dark sector models» that can be coherent to fit the increasingly tight straitjacket of facts placed on dark matter by new data. But the downside is that this creeping flexibility makes them very difficult to definitively test.

«With a dark sector, you’re free to dream up almost anything,» says David Spergel, an astrophysicist at Princeton University. «Now that we’ve lost the guidance of the WIMP miracle, the space for available models is simply vast. It’s a space where we don’t know what the right choice is, and more hints from nature about where to go next are simply needed.»

Some physicists, following hints from nature, have abandoned WIMPs entirely. For example, ghostly particles called neutrinos are known to exist and come in three flavors. Although these three flavors aren’t massive enough to account for dark matter, the presence of mass opens the possibility of a fourth flavor—massive, so-called sterile neutrinos. «Almost all neutrino mass-generation mechanisms require the existence of sterile neutrinos, and it would be very easy for some of them to account for dark matter,» says Kevork Abazajian, a theorist at Irvine.

Another perennial dark horse dark matter candidate is the axion, a hypothetical weakly interacting particle first postulated in 1977 to explain and resolve an otherwise mysterious asymmetry in quantum interactions. To explain dark matter, axions must occupy a relatively narrow mass range and be much lighter than WIMPs. «If we don’t find WIMPs, theorists will simply shift their bets to axions,» says Peter Graham, a physicist at Stanford University.

Beyond WIMPs and dark sectors, sterile neutrinos and axions, there are even more exotic possibilities for dark matter, though they reside on the fringes of physics, including «primordial» black holes, extra dimensions, and the possibility that Einstein’s theory of gravity is somehow flawed.

Regardless of their preferred candidate, the big problem for many physicists confronting dark matter may not be that the concept will ultimately be viewed as invalid or entirely flawed—the observational evidence for dark matter is overwhelming. Instead, they worry that dark matter’s identity may simply prove irrelevant to the other great mysteries of physics and, therefore, no new path to understanding the true nature of reality.

«The desire for dark matter is not only to exist, but also to unravel other unsolved problems in the Standard Model,» says Jesse Thaler, a physicist at MIT. «Not every new discovery can be a revelation… where then suddenly theories fit together much better. Sometimes new particles just make you ask, ‘Who was looking for this?’ Do we live in a universe where every discovery leads to deeper, more fundamental ideas, or do we live in a different universe where some parts rhyme and reason and others don’t? Dark matter offers all possibilities.»