Galaxies rotate faster than predicted by their visible mass, indicating the presence of unseen 'dark matter'.

In the 1930s, Swiss astronomer Fritz Zwicky observed the Coma Cluster of galaxies and noted that individual galaxies were moving too fast to remain gravitationally bound within the cluster based on its visible mass. He coined the term 'dunkle Materie' (dark matter). Decades later, in the 1960s and 70s, pioneering work by American astronomer Vera Rubin and her colleagues provided more robust evidence. By studying the rotation curves of spiral galaxies, they found that stars and gas clouds at the outer edges were orbiting the galactic centers at unexpectedly high speeds. According to Newtonian gravity, objects further from the center should slow down, much like planets further from the sun. The only explanation for their sustained high velocities was that the galaxies contained far more mass than could be seen, distributed in a vast, spherical 'halo' extending far beyond the visible stars.