Galactic Recession

This graph shows the correlation between a sample of galaxies recessional velocity and their distance from earth.

Why are the data points not all on the line?

This is because all galaxies have some additional residual motion in addition to the pure expansion. This is referred to as the "cosmic velocity dispersion" or "cosmic scatter" and is probably due to the fact that the gas clouds that formed the galaxies all had some small additional motion of their own.