In 1929 Edwin Hubble discover that wavelength of light is longer for more distant galaxies. Light wave stretch as it moves further. It becomes longer and redder. It is called Red Shift. By observing red shift scientists found that in large scale all galaxies around us are moving away. That means our universe is expanding. Some galaxies which are close together can attract each other gravitationally. But they are also moving away from other galaxies in the universe. So attraction of galaxies is a local event where expansion of universe is a universal event. If galaxies are moving away, at some point they must have been together. Everything we see today must have come from a single point. It introduces the idea of Big Bang theory.