Dark matter and its impact on cosmology have puzzled physicists for nearly a century. At Perimeter Institute, two researchers are trying to better understand how one potential dark matter candidate, self-interacting dark matter (SIDM), could impact how cosmic structures evolve.
In a paper published in Physical Review Letters on November 24, 2025, James Gurian and Simon May reveal a new code to study SIDM’s relationship to galactic evolution and enable research into a range of particle interactions that were practically inaccessible using previous methods.
Dark matter that dances alone
SIDM is a dark matter candidate whose particles can collide and bounce off one another but don’t interact with baryonic matter, the “ordinary” matter that we can observe, like protons, neutrons, and electrons. SIDM collisions conserve energy in what's called an elastic self-interaction. This has implications for dark matter halos – cosmological structures that are theorized to play a key role in star formation and galaxy evolution.
"Dark matter forms relatively diffuse clumps which are still much denser than the average density of the universe,” says Gurian, a Perimeter postdoctoral fellow who co-authored the study. “The Milky Way and other galaxies live in these dark matter halos.”
The nature of SIDM helps drive a process called gravothermal collapse within these dark matter halos. Gravothermal collapse is a consequence of the unintuitive fact that gravitationally-bound systems get hotter when you remove energy, rather than cooling.
“You have this self-interacting dark matter which transports energy, and it tends to transport energy outwards in these halos,” says Gurian. “This leads to the inner core getting really hot and dense as energy is transported outwards.” The endgame of this process is a gravothermal core collapse.
Mapping the structures formed by SIDM is challenging, but physicists have devised a few approaches, each of which works best for specific densities of matter.
“One approach is an N-body simulation approach that works really well when dark matter is not very dense and collisions are infrequent. The other approach is a fluid approach – and this works when dark matter is very dense and collisions are frequent.”
“But for the in-between, there wasn’t a good method,” Gurian says. “You need an intermediate range approach to correctly go between the low-density and high-density parts. That was the origin of this project.”
Gurian and his co-author Simon May, a former Perimeter postdoctoral researcher and now ERC Preparative Fellow at Bielefeld University, developed a code to address this missing middle. The code, KISS-SIDM, is faster and more accurate than previous codes that came before it and is publicly available for researchers to use.
“Before, if you wanted to check different parameters for self-interacting dark matter, you needed to either use this really simplified fluid model, or go to a cluster, which is computationally expensive. This code is faster, and you can run it on your laptop,” says Gurian.
“There has been considerable interest recently in interacting dark matter models, due to possible anomalies detected in observations of galaxies that may require new physics in the dark sector,” says Neal Dalal, Perimeter research faculty.
“Previously, it was not possible to perform accurate calculations of cosmic structure formation in these sorts of models, but the method developed by James and Simon provides a solution that finally allows us to simulate the evolution of dark matter in models with significant interactions,” says Dalal. “Their paper should enable a broad spectrum of studies that previously were intractable.”
Understanding the core collapse process also intrigues physicists because it could have observable implications for black hole formation. But the details of how the process ends is an open question in physics, says Gurian. This code is a step towards answering it, he says.
“The fundamental question is, what’s the final endpoint of this collapse? That’s what we’d really like to do -- study the phase after you form a black hole.”
About PI
Perimeter Institute is the world’s largest research hub devoted to theoretical physics. The independent Institute was founded in 1999 to foster breakthroughs in the fundamental understanding of our universe, from the smallest particles to the entire cosmos. Research at Perimeter is motivated by the understanding that fundamental science advances human knowledge and catalyzes innovation, and that today’s theoretical physics is tomorrow’s technology. Located in the Region of Waterloo, the not-for-profit Institute is a unique public-private endeavour, including the Governments of Ontario and Canada, that enables cutting-edge research, trains the next generation of scientific pioneers, and shares the power of physics through award-winning educational outreach and public engagement.
