Hubble's law applies to all galaxies or clusters sufficiently distant from one another that gravitational forces are negligible. According to the law, these galaxies are flying away from each other at tremendous speeds as the fabric of space they occupy stretches, such that the greater the distance between any two galaxies, the greater their relative speed of separation. In other words, the expansion of the universe is roughly uniform. This empirical finding strongly supports the theory that the universe began with an explosive big bang (see cosmology).
Hubble's law was deduced from observations that indicate that the more distant a galaxy, the greater its red shift and hence the greater its velocity relative to the Milky Way. The fact that all other galaxies (with the exception of M31, the Andromeda Galaxy) seem to be receding from the Milky Way does not imply that there is anything special about our position in space. Because the expansion of the universe is approximately uniform, it would appear to an observer in any galaxy that all other galaxies, including the Milky Way, were receding from the observer's galaxy.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.