Over the centuries, our understanding of the cosmos has grown by leaps and bounds. But it wasn’t until relatively recently that astronomers discovered that around 85 percent of the matter in the universe takes on a bizarre, foreign form. And just like detectives in the best crime-thrillers, astronomers must hunt for this elusive dark matter by searching for subtle clues, sifting through convoluted evidence, and, critically, identifying likely suspects.
“Dark matter is the name scientists have given to the particles which we believe exist in the universe, but which we cannot directly see,” theoretical physicist Johar M. Ashfaque tells Astronomy. Ashfaque is now a data scientist at East Kent Hospitals University NHS Foundation Trust in the U.K., but he first became interested in the dark-matter mystery while earning a Ph.D. focused on string theory at the University of Liverpool.
“This material appears to have mass, but it does not appear to absorb or emit any electromagnetic radiation,” he explains. “Given the fact that it does not send us any light, it is not difficult to understand that it has been hard to discover anything about the nature of these mysterious particles.”
Yet, despite this, we aren’t completely blind when it comes to dark matter; scientists have been able to shed light on the problem. The majority of our knowledge about dark matter comes from the fact that, although it doesn’t interact with light, it does interact with gravity — that’s how we know it really exists.
Specifically, the gravity of regular (baryonic) matter locked up in galaxies’ stars, planets, and gas isn’t strong enough to bind those galaxies together as tightly as they are observed to be. Without dark matter, astronomers would see stars on the outskirts of the galaxies orbiting much slower than those orbiting near the center. Yet, starting with observations of the Andromeda galaxy made by Vera Rubin and Kent Ford in the late 1970s, that’s not what researchers found. Visible matter at the fringes of galaxies actually orbits faster than it should, suggesting galaxies contain an invisible form of non-baryonic matter that researchers have dubbed dark matter.
Astronomers’ meticulous identification of the indirect effects of dark matter is akin to gathering clues. And just as that process works for police or private detectives, making sense of these clues helps scientists identify the likely perpetrators.
Rounding up the usual suspects
Most of the probable candidates for dark matter may be hypothetical particles, but their theorized existence is still based on real evidence. And that evidence comes from many eyewitness accounts that astronomers have compiled over the years, helping them sketch out some possible mugshots.
The likely sources of dark matter divide into two rough categories: cold dark matter (CDM) and hot dark matter (HDM). The category names don’t refer to temperatures, though; instead, they refer to speed, with a ‘cold’ particle being one that is moving well below the speed of light.
One strong theoretical CDM candidate for dark matter is weakly interacting massive particles, or WIMPs. These hypothetical particles are dubbed “weak” because they only interact with themselves through two of the four fundamental forces: the weak nuclear force and gravity. In other words, WIMPs basically ignore other matter.
Joel Primack, a emeritus physics professor at the University of California, Santa Cruz, tells Astronomy that “Heinz Pagels and I were the first to point out in our 1982 article in Physical Review Letters that the lightest supersymmetric partner particle is a natural candidate to be dark matter . … And the lightest supersymmetric partner particle would be a WIMP.” But even if the profile fits, astronomers still need to catch WIMPs red-handed to prove they’re involved.
Though direct evidence for WIMPs as dark matter remains elusive, researchers have already found some indirect evidence. Namely, Primack says, “Gamma rays from the center of the Milky Way nicely fit the predictions of WIMP annihilation.” When two WIMPs come in contact with one another, they don’t just bounce off each other or pair up; they completely destroy one another in a powerful burst of energy. Yet, he adds, “there are other possible explanations [for the gamma rays].”