For centuries, scientists have been hunting for an invisible glue that many believe holds our cosmos together. They have yet to observe this “dark matter,” but there are convincing signs it exists; the effects of the gravity it creates on nearby objects can be observed, for example. Learning more could be the key to solving the deepest mysteries of the universe.
And astronomers may have just unlocked a valuable clue that could help them, finally, observe the material. Most scientists theorize that dark matter comprises roughly 85 percent of the matter in the universe, but scientists believe that it’s made of stuff invisible to our telescopes. Another reason it’s hard to observe, some scientists believe, is that the mysterious substance could be what they call “collisionless,” meaning its particles interact with each other and other material indirectly.
However, a recent study published in Physical Review Letters is turning that assumption on its head. The study authors propose a new type of dark matter that they claim could be “self-interacting.” Unlike the traditional conception of dark matter, these particles would not only interact with each other, but also other material around them.
In everyday human terms, self-interacting dark matter behaves like a crowd of people who deliberately bump into each other instead of quietly avoiding others in the group, says Hai-Bo Yu, PhD, a physicist at the University of California, Riverside, who was one of the study’s authors. Rather than being made of collision-free and “cold” (or slow-moving) material, Yu’s research suggests that self-interacting dark matter could explain some kinds of structures we do see with telescopes.
However, one challenge in proving Yu’s theory is the fact that we don’t know what particles dark matter is made of, explains Yonatan Khan, PhD, a physicist at the University of Toronto, who did not participate in the study. “We don’t know how much it weighs, we don’t know whether it interacts with ordinary matter through forces of open gravity, and we also don’t know whether it interacts with itself,” he says.
Khan says Yu isn’t the first to propose this form of matter, but the reason it hasn’t been proven is because of observation issues: “Part of the challenge has been trying to find the right astrophysical observables that really focus on the signatures of self-interacting dark matter,” he says.
But if dark matter could interact with itself, Yu suggests that when the particles collide with each other, they would produce energy so immense that it could create dense, compact cores. However, our supercolliders, or machines that smash particles together at eye-watering speeds, aren’t yet powerful enough to find the particles associated with dark matter. But if we eventually do find these cores, Yu says they could help explain three important cosmic puzzles—pulling back the curtain on the inner workings of our universe.
Scars on the universe
A stream of stars located roughly 25 light-years away called GD-1 appears “scarred,” likely from the energy of an object that crashed through it long ago. In 2025, Yu was part of a team that wrote a paper in the Astrophysical Journal Letters, suggesting that this unknown object would have had to be extremely dense and made of a self-interacting form of dark matter. The team stated the scar left on the stream is too big to otherwise explain with the density of cold dark matter.
Yu adds that more observations of different stellar streams are needed to verify the work. And this isn’t some distant dream—studying these bands of stars could be possible in the next five years as more wide-field telescopes come online (such as the forthcoming Vera C. Rubin space telescope, located in northern Chile). “Then we might be able to exclude other explanations,” such as whether the gravitational well [or strength] of a galaxy changes over time and morphs stellar streams, Yu says.
Bizarre gravitational effects
Astronomers often take advantage of “magnifying glasses” in space. These aren’t the iconic detective tools you might’ve played with as a child; rather, they’re phenomena produced when one very dense object—such as a galaxy—magnifies the light of an object behind it (from the perspective of an astronomer on Earth), allowing them to see faint stars or galaxies otherwise too difficult to spot with a telescope alone.
And self-interacting dark matter may be key to learning more about these magnifying glasses. Yu suggests there is an ultra-dense dark matter object in one of these gravitational lens systems, known as JVAS B1938+666. However, he also acknowledges recent research that suggests that many factors could be contributing to the lens—including the gravity of a nearby elliptical (oval) galaxy.
The challenge in proving or disproving the theory is that the lens is quite far away and it’s difficult to figure out what is happening. “It’s also particularly hard in astronomy, because [the lens] occurs far away from us. You cannot reproduce it in a laboratory.”
Unusual star clusters
There is a mysterious dwarf galaxy, smaller than our Milky Way, that American astronomer Harlow Shapley discovered in 1938. Known as the Fornax satellite galaxy—as it’s a “satellite” of our own—the galaxy hosts six star clusters, including the strange Fornax 6. The bizarre star cluster is dimmer than expected, and also has an irregular shape.
While some astronomers say the gravity from Fornax 6’s host galaxy might be tugging on the stars, Yu instead refines the discussion by suggesting that a clump of dark matter may be trapping stars into the cluster. But again, newer observatories will be needed to find similar clusters and to add validity to his research, he warns.
Yu and Khan agree that with more evidence coming in, self-interacting dark matter could be responsible for these three cosmic mysteries—and might reveal more about the nature of the universe, including particles here on Earth.
“I don’t think this study is the end of the story at all,” Khan says.
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