Why dark matter's mysteries persist after decades of searching

One mile beneath a mountain in Italy, scientists at the Gran Sasso National Laboratory fill a particle detector with liquid xenon, hoping to observe evidence of dark matter. The idea is that, free from cosmic rays that interfere with these sorts of experiments aboveground, the lab will eventually detect invisible particles that do not interact with light by mapping how those particles collide with the xenon in the experiment — almost like a group of pool balls that shoots out in all directions when struck by a cue.

Around one billion of a certain group of particles called weakly interacting massive particles — or WIMPS, for short — are expected to pass through this detector per second. But so far, none of them have collided with dark matter, said Dr. Abigail Kopec, an Assistant Professor of Physics & Astronomy at Bucknell University in Pennsylvania, who works with the collider’s data. However, there are many experiments currently being run in the dark matter hunt, each specifically geared to detect it based on what we know about how it behaves in the universe. 

If dark matter is discovered through one of these experiments or another that hasn’t yet been dreamed up, it could essentially shed a light on an entire hidden universe that for now remains a mystery, said Dr. Tracy Slatyer, a theoretical particle physicist at the Massachusetts Institute of Technology.

“We could unveil that whole invisible scaffolding of the universe, to map it out, not just by its gravity, but by now seeing it directly in the right kind of light,” Slatyer told Salon in a video call. “The reason to understand what dark matter is to understand the universe.”

The first evidence that dark matter existed is traced back to the 1930s, but it became even more clear that some invisible mass was acting on the gravitational forces of the universe in the 1960s. That's when astronomers noticed that galaxies were moving too fast given the amount of light they were observing coming out of them. In other words, some other form of matter besides what we could observe was influencing their gravitational pull. Throughout the decades, numerous observations of how dust, gas, and ripples in the cosmic microwave background, or the leftover radiation from the primordial plasma of the universe, moved indicated that dark matter exists. 

“All of this has come down to the conclusion that gravitationally, something is pulling on the luminous matter, the matter we can see, that does not interact with light,” Kopec told Salon in a video call. “Dark matter makes up about 25% of the universe … Right now this is a huge gap in our understanding of the universe.”

Although astrophysicists have been able to calculate very precisely that the universe is made up of 26.8% dark matter, its true characteristics remain elusive. This is difficult to puzzle out because, as mentioned, dark matter does not interact with light and it does not seem to decay over time — but it does have gravity. It is clearly present in our galaxy, but is found in higher concentrations in some other galaxies called dwarf steroidal galaxies. And when two galaxy clusters collide with each other, clouds of dark matter in them pass straight through each other, without slowing down.

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These clues serve as the basis to design experiments. Currently, the two most popular designs involve experiments like Kopec’s in Italy that try to determine whether dark matter consists of WIMPs, versus experiments investigating whether dark matter is an axion, a hypothetical elementary particle proposed in the theory of quantum chromodynamics (QCD.)

The WIMP idea is related to another idea that could explain dark matter called supersymmetry. This is essentially the idea that there is an underlying symmetry in the universe, and for every particle we know about there is a partner particle (yet to be found) that could constitute dark matter, Slatyer said. The names for these speculative particles often tack on an S to the names of known particles: selectrons contrast electrons, the squark is the inverted twin of the quark, and so on.

However, in more than 10 years of observations there has been no evidence to support this idea, even using the Large Hadron Collider as some had hoped.

“This class of ideas has become less popular because when we turned on the Large Hadron Collider, we did not see evidence of supersymmetry,” Slatyer told Salon in a video call. “This is still a viable possibility, but one of the things that happened after the LHC didn't find this was that it prompted people to realize that this was never the only possibility.”

The idea behind the QCD theory is that dark matter could be thousands and thousands of times lighter than any of the particles we currently know about and acts more like a wave. This is an attractive hypothesis because it would also solve something called the strong CP problem in the standard model of cosmology, said Dr. Ciaran O’Hare, a particle astrophysicist at the University of Sydney who studies dark matter. This is essentially a tension in the model where something doesn't add up when examining the nuclear force that binds together protons, neutrons and other particles. 

“If dark matter were a QCD axion, it would essentially be invisible to us,” O’Hare told Salon in a video call. “We would be flowing through it, but we wouldn’t notice most of the time and would have to build very specific experiments to see that."

Although technology has advanced since the first axion detectors went online in the 1980s, the challenge with most of them is that they test each mass possibility of dark matter one at a time, Kopec said. 

Scientists were able to detect a form of "hot dark matter" when they discovered neutrinos, enigmatic particles that are so small they have next to zero mass. Unsurprisingly, this makes detecting this particle extremely challenging to study. In an experiment colliding particles in a 5-by-5 foot detector, it took Kopec’s team two and a half years to identify just 11 neutrino collisions. Still, others are testing whether the rest of dark matter exists as “sterile” neutrinos, meaning particles that don’t interact with other visible particles. Although this is still a plausible hypothesis, these particles likely would not constitute the majority of dark matter in the universe. 

Another leading theory is that dark matter could be hiding out in primordial black holes, which were created early in the universe. The challenge with finding evidence to support this idea is that scientists have determined that for this to be true, it would have to be black holes the size of about an asteroid, which are difficult to stumble upon given the scale of the universe, O’Hare said.

“Observing black holes with the mass of an asteroid is just unbelievably difficult,” O’Hare said. “We have ideas but it will take a bit of time to really develop those ideas and flesh them out. I would say maybe in the next five years if we're really lucky we will close that gap and have either seen the thing or ruled it out completely for black holes.”

The field has been searching for dark matter for decades, but each unsuccessful experiment is hopefully one step closer to finding dark matter. Or it could turn up in one of the current experiments tomorrow. Scientists remain optimistic that we'll might turn up evidence of dark matter in the next decade. Another possibility is that we may never find it and that doing so involves some physics we don’t yet understand or cannot observe, Slatyer said.

“It could be that this idea that we’re going to test this experimentally is just a false hope,” Slatyer said. “But at the same time, given what we know, dark matter could be a new particle that is lighter than any of the particles we know about, something that is being produced all the time around us, particles that are continually flying through the room — and you just need to put up a sensitive detector and you will find them.”