The Big Bang theory of the Universe was first suggested by a Belgian priest Georges Lemaître in the 1920s. A similar model of the universe had been proposed by a Russian scientist Alexander Friedmann in 1922 (he actually proposed 2 theories, -the theory of the oscillating universe & the theory of the expanding universe (the Big Bang theory).
The theory became even more popular right after Edwin Hubble’s demonstration of the continuously expanding universe in 1929. And finally, the Big Bang became the mainsteam scientific theory after the discovery of cosmic microwave background radiation by Arno Penzias and Robert Wilson in 1965.
The Big Bang theory describes the universe as originating in an infinitely tiny and dense point (or singularity) 13.7 billion years ago, from where it has been expanding ever since. Singularities are thought to exist at the core of black holes. Black holes are areas of intense gravitational pressure. The pressure is thought to be so intense that finite matter is actually squished into infinite density.
However, singularity itself appeared out of nowhere for reasons unknown. According to the many experts, space didn’t exist prior to the Big Bang. The singularity didn’t appear in space; rather, space began inside of the singularity. Prior to the singularity, nothing existed, not space, time, matter, or energy – nothing.
Yet there are lots of unanswered questions and …more theories! One of the exciting ones is the idea of Niayesh Afshordi, from the Perimeter Institute in Canada, and his two colleagues. They claim that the universe (that we know of was never inside the singularity) was instead formed from the debris of a star in the four dimensional universe that collapsed into a black hole.