The Big Bang Theory is the most acclaimed theory proposed by Georges Lemaitre in 1930s and later strongly evidenced by George Gamow, Which explained the modern Big Bang theory.
According to this theory, At the beginning, all matter forming the universe existed in one place in the form of a tiny ball (singular or primeval atom) with an unimaginably small volume, infinite temperature and indefinite density.
Around 15 billion years ago, a vast explosion occurred and the primeval atom disintegrated giving rise to space and time and the expansion of the universe that still continues till today and will be further.
The expansion of matters further formed galaxies and these galaxies are continuously expanding and moving away from each other in an accelerated velocity and in the process cooling the universe.
The Big Bang Theory or the expanding universe is now proven by the discovery of the Cosmic Microwave Background Radiation (CMBR) and Wilkinson Microwave Anisotropy Probe (WMAP).
Another important astronomic observation called Red Shift also proved the expansion of universe.
The recent experiments at the Large Hadron Collider (LHC), built by the European Organization for Nuclear Research beneath the Franco-Swiss border near Geneva, Switzerland, aims to recreate conditions just after Big Bang to have a better understanding of the phenomenon.