In August 2006, NASA released an image that showed some of the best evidence of dark matter. A composite image of galaxy cluster 1E 0657-556, colloquially known as the “bullet cluster,” it shows the remnants following a massive collision of two galaxy clusters.
In the image, nebulous clumps colored pink and blue occupy the focal point against a blanket of galaxies. These clumps are the galaxy clusters’ collision point. The pink clumps, at dead center, are hot gas, or normal matter, but the blue clumps, at the pink’s edges, are regions where scientists found the most mass.
In essence, “dark matter and normal matter have been wrenched apart,” NASA said in its commentary about the image.
“If we look at where the mass is using gravitational lensing, which allows you to see the gravitational pull of an object, it’s not where the hot gas is,” Matthew Citron told the crowd gathered at Sudwerk Brewing Co. for the May edition of Astronomy on Tap. Behind him, a projected image of the bullet cluster occupied a screen. “And that’s exciting evidence for the existence of this thing called dark matter.”
An assistant professor in the Department of Physics and Astronomy at UC Davis, Citron is a particle physicist. While almost the complete inverse of astronomy and cosmology, fields concerned with the largest objects in our universe, particle physics aims to answer similar questions but from a different vantage.
“Particle physics is the study of the very smallest, fundamental building blocks of our universe,” Citron said. “But by understanding those, we can understand the conditions that existed just after the Big Bang, when the universe was in its hottest, densest state, and then evolved over time into what we see today.”
“Particle physics is really key to understanding our universe today,” he added.
This includes understanding dark matter, something Citron is keen to unravel.
A dark sector of particles
The search for dark matter has been ongoing for nearly 100 years. Theories abound about its makeup. Originally, scientists thought it could be a single particle, but one idea that’s gained traction, and Citron’s attention, is the idea of a dark sector — a complete dark side of our universe with a host of dark particles and forces.
“Dark sector theories are not extra universes or parallel dimensions or anything like that,” Citron explained. “Dark sectors, most likely, are a few extra particles that are either very unstable or that don’t interact very much.”
If this is the case, Citron said, there should be a particle that links our universe’s normal matter to the dark sector.
“There must be at least one particle that talks to both us and the dark sector and this is what we call a portal particle,” he said. “But it’s very likely that this particle is way too unstable or way too feebly interacting to see it in nature.”
That’s where Citron and his colleagues enter the equation. They’re using particle accelerators, such as the Large Hadron Collider, or LHC, to create the conditions necessary to coax these dark particles into existence, even if for the briefest of moments.
Particle detectors big and small
At 27 kilometers in circumference and placed about 100 meters underground, the LHC is a feat of engineering. Thousands of superconducting magnets guide its particle beams through tubes to collision points, such as the Compact Muon Solenoid detector, or CMS.
The CMS experiment is a general-purpose detector, designed to conduct different kinds of physics. However, searching for dark sector particles with CMS is often very challenging. While the LHC may create dark sector particles in its collisions, experiments like CMS weren’t necessarily designed to detect them.
“There are dark sector particles that can be produced that are completely invisible to that detector,” he added. “So that kind of begs the question, ‘What can we do?’”
That question led to the creation of FORMOSA, a dedicated detector fabricated and installed by Citron and UC Davis students at the LHC. Positioned 500 meters away from where proton collisions occur, FORMOSA is designed to detect millicharged particles, proposed subatomic particles that have a tiny fraction of an electron’s charge (somewhere between 0.1% and 10%).
FORMOSA collected data on billions of collisions between early 2024 and late 2025. Citron and his colleagues are now sifting through the data for evidence of millicharged particles.
“Particle physics aims to connect the very small to the very big by uncovering the structure of particles and their interactions,” Citron said. “Cosmologists and astrophysicists can then connect that to the universe at large.”
“Discovering a dark sector,” he added, “would really revolutionize everything we know about our universe.”
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