If you ask any physicist to identify the biggest mystery in their field, dark matter would probably be toward the top of the list. It makes up about 80 percent of the mass of the universe, but because it doesn’t emit light or energy, it’s proven nearly impossible to detect since it was first proposed in 1933.

So how do we even know it’s there? Gary Bernstein, a Penn professor of astronomy and astrophysics, compared it to the discovery of Neptune. Before astronomers ever even saw Neptune, they realized it had to be there because of the pull it exerted on Uranus. The influence of dark matter on other objects in the universe has many scientists convinced that it must exist.

One of the strongest lines of evidence for the existence of dark matter is the way galaxies spin: Based on the gravitational force of everything we’ve observed in the universe, all the stars and other visible matter at the outskirts of galaxies would fly apart into space without the existence of dark matter to exert an additional gravitational pull.

Dark matter also helps explain things like how galaxies clumped together when the universe was forming, and why there are small ripples in the cosmic microwave background, the leftover radiation from the big bang.

One type of measurement that lends a huge amount of support to the idea of dark matter is gravitational lensing, which looks at how light coming toward us bends as matter exerts gravitational force on it.

One of Penn’s specialties when it comes to probing dark matter is a technique called weak gravitational lensing. Using the Dark Energy Survey and other information, Bernstein, Professor Bhuvnesh Jain, research scientist Mike Jarvis and postdocs Eric Baxter and Kathleen Eckert look for the subtle bending of light that might be produced by dark matter.

“By measuring correlations between lensing and galaxies,” Baxter says, “we’re probing the distribution of matter in the universe in many different ways. By measuring how it evolves over time, we can learn things about dark matter and the geometry and structure of the universe.”

Bernstein first began working on this technique after completing his graduate studies in the cosmic microwave background in 1989. Anthony Tyson, a physicist at Bell Labs, had recruited Bernstein to work on what was, at the time, a brand new way of investigating dark matter and dark energy, which together make up 95 percent of the universe.

“When I was trying to decide what to do after my Ph.D., I was a little averse to trying to become world’s expert in some corner,” Bernstein says. “But 95 percent of the universe is not a corner. There aren’t many places these days where you have as good a chance to really open a new chapter in the laws of physics. There’s some appeal to be really answering a fundamental question.”

Jain and Baxter work on investigating the shapes and boundaries of dark matter halos, clouds of dark matter that extend far beyond the reach of the furthest stars in galaxies and galaxy clusters.

They are also working on mapping dark matter using the hot and cold spots of the cosmic microwave background, which are also subtly distorted by lensing. This allows them to trace the history of dark matter back to some of the earliest moments of the universe.

“The most distant galaxies we use as wallpaper are more than halfway to edge of the observable universe,” Jain says. “The cosmic microwave background is more than 99 percent to the edge. By combining these measurements, we can learn how dark matter fluctuations existed much further back in time.”

Jose Maria Diego, a visiting scholar at Penn, works on different kinds of gravitational lensing, such as strong lensing and microlensing, to map the distribution of dark matter in galaxy clusters, which are the objects that contain the largest amounts of dark matter in the universe. Unlike weak gravitational lensing, which is just a slight stretching of light, strong gravitational lensing produces more drastic optical tricks, like making two copies of the same galaxy in one image.

Using microlensing, which amplifies light from individual objects, Diego is also investigating whether or not primordial black holes could explain dark matter.

One of the leading candidates for dark matter are weakly interacting massive particles, or WIMPs. Associate Professor Elliot Lipeles, who is part of the ATLAS collaboration at the Large Hadron Collider, works on experiments that are trying to create these WIMPs, in particular through its interaction with a Higgs boson.

Because the dark matter itself is undetectable, Lipeles uses momentum conservation to search for the existence of WIMPs. If two particles smash together in the collider and the particles they produce don’t add up to the same total momentum as the original particles, it’s possible that some of that momentum escaped in the form of dark matter.

“It’s one of these deep mysteries that could point towards a better understanding of the universe as a whole,” Lipeles says. “It’s the type of question where you don’t even know the impact because you don’t even know what the answer looks like. You can’t figure it out by just doing math. You have to actually go out there and check for it.”

Associate Professor Chris Mauger, who works primarily on studying a different type of invisible particle called a neutrino, runs experiments that use indirect detections to try to learn more about dark matter.

Mauger explains that it’s possible that these dark matter particles could get stuck in gravitational wells, such as the sun or the core of the earth. And it’s also possible that when two of these dark matter particles collide with each other, they could annihilate into neutrinos. If one of the experiments Mauger works on, such as the upcoming Deep Underground Neutrino Experiment, detect an excess of neutrinos coming from places like the sun, it would hint toward the presence of dark matter.

Professor Josh Klein is involved in direct detection experiments, such as MiniCLEAN, which use underground detectors filled with a material such as liquid argon to detect the scattering caused by a particle of dark matter smashing into another particle and producing tiny amounts of sound or light. Klein described these types of experiments as “dark matter telescopes.”

“We talk a lot about the overlap between fundamental particle physics and astrophysics,” Klein says. “Even when I was a grad student I kind of walked the line between astrophysics and particle physics. Dark matter is exactly this intermediate regime where you’re doing a particle physics experiment to answer an astrophysical or cosmological question. It’s also very challenging in a way that is amenable to a lot of creativity.”

When it comes to studying dark matter, many agree that one of the most important skills to have is patience. Despite decades-long investigations, Bernstein says we’re still very much in the dark about what it could be.

“It's like watching a thriller,” Diego says. “There’s suspense. Whenever we find what's going on, something is going to change dramatically. I have no idea how the world will be in 100 years, but it may have changed a little bit because of what we learn about dark matter.”

Photo: Dark Energy Camera (DECam) on Blanco Telescope (Reidar Hahn, Fermilab)