It is often said that we have not yet discovered any dark matter particles. That is not completely right. We didn't discover the particles that make up cold dark matter, but we did discover neutrinos. Neutrinos have mass and don't interact strongly with light, so they're a form of dark matter. While they do not solve the mystery of dark matter, they play a role in the shape and evolution of our universe.
The possible explanations for the nature of dark matter. Photo credit: G. Bertone and T. M. P. Tait
From the evidence we have for dark matter, like the accumulation of galaxies and gravitational lenses, we know that most dark matter must be cold. That said, it's likely made up of heavy particles. A number of possibilities have been suggested, from exotic particles called axions to tiny primordial black holes. No such solution has yet been found. While most dark matter must be cold, other dark matter that is warm or hot could also play a role.
Neutrinos come in three flavors. Photo credit: IceCube Collaboration
Neutrinos are a form of hot dark matter. The temperature of a material is determined by the speed of its particles. Since neutrinos move almost at the speed of light, they are a form of hot matter. For a long time it was believed that neutrinos were massless and therefore not part of dark matter. Then in the 1990s they were found to have a tiny amount of mass. Its mass is so small that we don't know what it is. We only know that neutrinos have mass because the state of a neutrino can change over time through a process known as oscillation. This would not be possible if they were massless and moving at the speed of light.
So neutrinos are part of dark matter, but what role do they play? That is the question recently investigated in the Astrophysical Journal. The team performed computer simulations on how neutrinos interact on a cosmic scale. Not knowing the mass of the neutrinos, they created a simulation in which they can vary the mass to study different results. They found that, while neutrinos tend to clump with galaxies, they actually serve to prevent the extent of clustering from cold dark matter. The degree of disability depends on the mass of the neutrinos.
Previous studies have shown how neutrinos can affect cosmic evolution, but this study shows how neutrinos can affect cold dark matter. Further research could even enable astronomers to use galactic clusters to determine the mass of neutrinos and thus use the most massive objects in the universe to measure particles with the smallest mass. It's a hot idea that would be pretty cool.
Reference: Yoshikawa, Kohji, et al. "Cosmological Vlasov-Poisson simulations of structure formation with relic neutrinos: Nonlinear cluster formation and the neutrino mass-ko." The Astrophysical Journal 904.2 (2020): 159.