Ultra-Strongly Self-Interacting Dark Matter: From Phenomenology to Astrophysical Observables

For decades, astronomers have mapped the universe with a model that works remarkably well on the grandest scales, describing a cosmos filled with invisible matter that holds galaxies together. Yet, when they zoom in to look at the smallest structures, the details become messy. Some galaxies spin in ways that suggest their centers are less dense than expected, while others seem to have too much mass packed into a tiny core. At the same time, telescopes have recently spotted strange, compact, red objects in the very early universe, hinting that massive black holes formed much faster than standard theories allow. These puzzles suggest that the invisible matter making up most of the universe might not be a single, uniform substance, but rather a complex mixture with its own internal rules and interactions.

A team of researchers at the University of California, Santa Cruz, has proposed a new way to resolve these conflicting clues. They suggest that dark matter is not just one type of particle, but a two-component system. In their model, the vast majority of dark matter behaves moderately, interacting with itself just enough to smooth out the centers of galaxies and explain the rotation speeds of dwarf galaxies. However, a tiny, sub-percent fraction of this dark matter is "ultra-strongly" self-interacting. This rare component is so eager to collide with its own kind that it can collapse rapidly under its own gravity, potentially seeding the massive black holes seen in the early universe. The researchers built a mathematical framework to show how these two types of particles could have coexisted since the beginning of time without breaking the laws of physics or contradicting what we see in the sky today.

To test this idea, the scientists constructed a scenario where the two types of dark matter particles interact through a force carried by a light particle called a dark photon. They calculated how these particles would have behaved in the hot, dense environment of the early universe, tracking how their numbers changed as the cosmos expanded and cooled. Their calculations showed that the two species could naturally settle into the right proportions to match the total amount of dark matter we observe today. Crucially, the model predicts that the ultra-strongly interacting particles would be heavy enough and rare enough to avoid disrupting the large-scale structure of the universe, while still being capable of collapsing quickly enough to form the seeds of early black holes. This collapse happens because the particles scatter off one another so frequently that they lose energy and sink toward the center of a forming galaxy, a process that can happen much faster than in standard models.

The researchers then checked their model against real-world observations to see if it held up. They looked at the rotation speeds of dwarf galaxies and low-surface-brightness galaxies, which are known to have cores that are less dense than expected. Their model successfully reproduced the range of speeds seen in these small galaxies, requiring the dominant dark matter component to have a specific interaction strength. At the same time, they ensured the model did not violate strict limits set by observations of massive galaxy clusters. In these clusters, where particles move much faster, the interaction between dark matter particles must be very weak to prevent the galaxies from being distorted in ways we do not see. The team found a narrow window of parameters where the dark matter is strong enough to affect small galaxies but weak enough to leave massive clusters untouched. This balance is achieved because the interaction strength depends on the speed of the particles, a feature that allows the model to satisfy both sets of constraints simultaneously.

Beyond the local universe, the team examined how their two-component system would affect the distribution of matter across the entire cosmos. They simulated the growth of cosmic structures to see if the presence of the ultra-strongly interacting fraction would leave a detectable imprint on the matter power spectrum, which describes how clumpy the universe is at different scales. They found that because the ultra-strongly interacting component makes up such a tiny fraction of the total dark matter, it does not significantly alter the large-scale structure of the universe. The small-scale cutoff in the distribution of matter remains consistent with observations of the early universe, such as the Lyman-alpha forest, which traces the distribution of gas between galaxies. This means the model can explain the rapid formation of early black holes and the "little red dots" seen by modern telescopes without erasing the delicate patterns of matter that astronomers rely on to understand cosmic history.

The study concludes that this specific two-component framework offers a coherent explanation for several long-standing mysteries. It provides a mechanism for the rapid assembly of black holes in the early universe while simultaneously solving the small-scale structure problems in nearby galaxies. The researchers identified a specific range of masses and interaction strengths where all these conditions are met, suggesting that the dark sector might be more diverse than previously thought. While the model relies on theoretical calculations and simulations rather than direct detection of the particles, it demonstrates that a small, ultra-strongly interacting subcomponent can coexist with a dominant, moderately interacting species. This arrangement allows the universe to look the way it does today, from the slow rotation of dwarf galaxies to the existence of ancient, massive black holes, all within a single, consistent physical picture.