That is, they are all receding from us, moving steadily further away. This relation was first discovered by Edwin Hubble, and is known as the Hubble Law . This is a surprising result. In fact, our galaxy does not possess a special repulsive force. For our purposes at the moment, it suffices to point out that the Hubble Law permits us to find distances to far-away galaxies.
Radial velocities are relatively easy to measure.