The force experienced by a mass such as a star cluster or dwarf galaxy travelling through an extended distribution of stars. The force arises because the gravitational effect of the mass as it moves causes a slight enhancement of the density of stars behind it, and the consequent increased gravitational influence acts as a drag on the mass. Dynamical friction is believed to be important in the merging of galaxies, and in the decay of the orbits of globular clusters around galaxies.