The [Big Bang] is a theory that known universe with all its galaxies, stars, and atoms exploded from a mass of a very small size and spread throughout space. Although this is the prevailing theory of cosmology, it is problematic and today, many scientists are abandoning this theory for an eternal universe.
The Big Bang was proposed because of an observation in the early 1930s of what is called red shift. Stars appeared to be shifted to a red color and it was theorized that this red shift was caused by the stars moving away from a central point at a very high or even accelerating speed. Since almost everything was red-shifted in our telescopes, it was assumed that the earth was near the point where all mass exploded to start the known universe.