Following (Zwicky 1937) we start with the virial theorem that relates the average kinetic energy ⟨K⟩ to the average gravitational potential energy ⟨U⟩ of bodies in a system (cluster, galaxy, etc) in the state of equilibrium: ⟨K⟩ = −1/2⟨U⟩, with brackets denoting an average value. We can measure radial velocities Vr of individual bodies in a system (of stars in a globular cluster or a galaxy, of galaxies in a cluster of galaxies) via spectroscopic means.

Note: Dynamical Mass Measurement of the Coma Cluster (Abell 1656)