Edwin Hubble observed that the galaxies in the universe are not stationary but are moving away from one another at a large speed.
According to Doppler Effect, if the source of light moves towards us, then the frequency of its light will increase and it will shift toward blue side of the visible spectrum. On the other hand, if the source of light moves away from us, then the frequency of its light will decrease and it will shift towards the red side of the visible spectrum. By studying the light emitted by a stat and observing the shift in frequency, we can find the velocity of the star’s motion with respect to the earth.
E. Hubble studied the light emitted by different galaxies and came to the conclusion that all galaxies are fast moving away from each other. He also showed that the speed with which they recede from one another was proportional to the distance between them; longer the distance faster is the motion. This shows that our universe is expanding.
According to E. Hubble, if galaxies are flying apart, they must all have been packed closely together in the beginning. In other words, all the galaxies must have started from a single point in space by a big bang (explosion).