How are galaxies except for the expansion of the universe moves due to mutual gravitational attraction, Hubble's law applies primarily to distant galaxies, whose speeds are due to expansion of the universe much larger than the speeds because of mutual attraction.

Hubble's law is usually written as:

v = H

_{0}D

where v is the speed of the removal observed (extragalactic) object, D its distance and H0 Hubble constant. To determine the value of the Hubble constant we need for a large number of galaxies to determine their removal speeds and distance. Speed of removal is relatively simple to determine from the Doppler effect in a range of sources, while finding the distance is significantly more complex problem. Therefore, the reliability in determining the Hubble constant is directly dependent on the accuracy of methods used to measure distances of galaxies.

Hubble's discovery was so unexpected that it is not immediately grasped its full significance. To easier demonstrate the Hubble's law, consider a balloon and draw on it a few dots. Slowly inflates it. As the balloon expands, so does the distance between each pair of dots increases. Imagine that you are one of these dots and look to other dots. From this point of observation you would seem that all the dots moving away from you. It does not matter which point you choose, each has the same view. Therefore, galaxies in the universe that spreads are like dots on the inflated balloon.