Even in blob-like systems where astronomers can't define a rotation curve (elliptical galaxies or galaxy clusters, for example), they can still use velocities to estimate masses.  In this case, astronomers treat these systems statistically and compute an averages for its members, and hence an approximate average kinetic energy.  A result of classical mechanics called the "virial theorem" relates the average kinetic and potential energies of a gravitational bound system, and the potential energy (since it is caused by gravity) is related to the total mass.  This analysis indicates that both elliptical galaxies and galaxy clusters reside in large dark matter haloes.