Dwarf ellipticals are the most common type of galaxy. The only galaxy visible with the naked eye is Andromeda, also called M31. The motion of galaxies within the cluster can be measured. There must be a lot of dark matter between the galaxies in order to keep them together. Velocity is the recessional velocity of a particular galaxy away from the observer.
Distance is the distance between the observer and the galaxy.
Edwin Hubble found that all of the galaxies have red-shifted spectra. He also found that the greater the distance to the galaxy, the larger the redshift, and therefore the larger the recessional velocity. This means that the universe appears to be expanding away from us in all directions at a constant rate. This may make it appear that we, on Earth, are the center of the universe. However, it can be shown that a uniform expansion of the universe in all directions will make it seem that the observer is at the center of the expansion, no matter where the observer is located.