What is the big bang theory and how does it explain the formation of the universe?
It is the theory that the universe began expanding rapidly from a singularity about 14 billion your ago and the universe continues to expand to this day, we can tell the universe is expanding because the further an object is from us the faster it is moving away from us. The universe may have been infinitely large at the Big Bang and it is getting infinitely larger.
In the 1920s Edwin Hubble made the observation that the universe had a prevailing red shift. This implied that the present observable universe had a beginning. The Big Bang theory is an attempt to explain the beginning of the universe implied by the evidence of an expanding universe.
Logically if the universe is expanding outwards then extrapolating backwards all the universe must have existed in one small space at the beginning. Einstein's theory suggests the existence of singularities where time and space does not exist. The entire universe is a small space would create a singularity because of the massive density of all the matter of the universe being in one place.
The theory is that this singularity with its mass exploded creating our present universe. The theory does not explain where or how the singularity came to exist, but does explain the beginning of the universe.
The previous theory of a steady state universe was abandoned because of the empirical evidence of an expanding universe that had a beginning. The present theories of an oscillating universe of big bangs and big crush is being seriously questioned by empirically evidence that rate of expansion of the universe is increasing. The evidence of an increasing rate of expansion indicates that the present observable universe will have an ending it will not recycle.