Abstract: Astrophysical observations spanning dwarf galaxies to galaxy clusters indicate that dark matter halos are much more diverse than predicted in the prevailing cold dark matter theory. In this talk, I will show that self-interacting dark matter (SIDM) provides a unified solution to a number of long-standing puzzles in astronomy and astrophysics, including the diverse galaxy rotation curves, the radial acceleration relation and the density cores in galaxy clusters. These results indicate that the inner dark matter halos of the galactic systems are thermalized due to dark matter self-interactions, a significant deviation from the cold dark matter theory.