Final answer:
Evidence for dark matter in the universe includes flat rotation curves of galaxies and the presence of unobserved ionized gas indicated by X-ray observations, hinting at a significant amount of invisible mass.
Step-by-step explanation:
Some evidence for dark matter includes the speed at which stars rotate around spiral galaxies. The rotation curves of galaxies, first noted by Fritz Zwicky and later by Vera Rubin, show that stars' velocities remain flat with distance from the galactic center, rather than decreasing as expected if only luminous matter were present. This unexpected rotation speed suggests a massive halo of dark matter extending beyond visible stars. Additionally, observations of X-rays from galactic clusters suggest the presence of hot, ionized gas in areas thought to be empty, further indicating dark matter's gravitational influence.