Collisionless dark matter refers to a type of dark matter that does not interact with itself or with ordinary matter through electromagnetic forces. It only interacts through gravity. This means that the particles of collisionless dark matter pass through each other and other matter without scattering, making it difficult to detect directly.
Self-interacting dark matter is a type of dark matter that can interact with itself through forces other than gravity. These interactions could involve scattering, annihilation, or other processes. The self-interactions can lead to observable effects such as changes in the distribution of dark matter in galaxies and galaxy clusters.
Collisionless dark matter leads to the formation of galaxies with cuspy cores and extended halos, as it does not dissipate energy through interactions. In contrast, self-interacting dark matter can lead to the formation of galaxies with cored density profiles, as the interactions can redistribute energy and smooth out the central density.
Evidence for self-interacting dark matter comes from observations of galaxy clusters and dwarf galaxies where the distribution of dark matter appears to be smoother and less dense in the center than predicted by collisionless dark matter models. These discrepancies suggest that some form of self-interaction might be at play.
Understanding the difference is crucial for developing accurate models of cosmic structure formation, interpreting astronomical observations, and ultimately identifying the true nature of dark matter. It helps scientists refine their theories and design experiments to detect dark matter particles and their interactions.