When our Universe came into existence there was NOTHING at all and Big Bang theory gives an explanation to this.
Big Bang theory has been developed by taking theoretical assumptions and observations together by George Lemaitre in 1927 by taking into consideration Albert Einstein's equations of general relativity and proposed and this model was being called "explosion" of a "primeval atom" which later came to be known as Big Bang theory.
In the year 1929 Hubble suggested that the Universe was expanding, contracting the infinite and is changing the "static universe" theory as proposed by Einstein. In "static universe" theory Einstein explained that our Universe is neither expanding nor it is contracting.
According to Big Bang theory after a long period of time the dense matter due to gravitational pull formed stars, galaxies, planets and other heavenly bodies which we find today.
Universe is in the form of dark energy today and near about 72% of the energy of Universe is in this form.
Misconceptions of Big Bang theory:
The most common misconception about Big Bang is that we assume that there was an explosion in the Universe but the experts say that there is an "expansion" and due to this "expansion" things in Universe are moving away from us.
There is no evidence of Big Bang theory as there is no explanation to the concept of what could have caused "Big Bang".
Big Bang theory gives no explanation to the existence of God. It is being believed that the Universe is a super natural creation by God. It gives no explanation to the fact whether God created Universe.