What discrepancy led Fritz Zwicky to infer the presence of *dunkle Materie* in galaxy clusters?

Galaxies within clusters were observed moving too fast to remain gravitationally bound by the gravity of the visible galaxies alone.

Decades before the precise work on spiral galaxies, Fritz Zwicky initiated the concept of unseen mass while studying much larger structures—galaxy clusters—in the 1930s. He performed velocity measurements on the galaxies constituting these clusters. His calculations demonstrated that the kinetic energy of the galaxies moving within the cluster was far too high for the visible gravitational pull exerted by the stars and gas components to keep the entire structure gravitationally coherent. To prevent the clusters from rapidly flying apart due to these high velocities, Zwicky concluded that a substantial amount of unseen mass, which he termed *dunkle Materie* (dark matter), must be providing the necessary binding gravitational force.