What term did Fritz Zwicky use for the unseen mass required to bind galaxy clusters?

*dunkle Materie*.

Fritz Zwicky, while observing galaxy clusters in the 1930s, was the first to rigorously quantify the mass discrepancy indicating that the gravitational pull derived from visible galaxies was insufficient to keep the clusters bound together given their observed velocities. To account for this missing gravitational influence, he inferred the presence of substantial, unseen mass. The specific term he used to describe this inferred component in German was *dunkle Materie*, which translates directly to dark matter. This established the initial conceptual foundation that gravitational effects in the universe often exceed the influence derived from matter that emits or reflects light.