Discoveries in astronomy and physics have shown a reasonable doubt that universe did in fact have a beginning. The big bang theory is an effort to explain what happened during and after that moment.
According to the standard theory, our universe came into existence as ‘singularity’ about 15 billion years ago and begun as an infinitesimally small, infinitely hot (about 1012K), infinitely dense which is also called singularities. They are through to exist at areas of intense gravitational pressure called ‘black holes’.
After its initial appearance, it apparently inflated (the big bang), expanded and cooled ranging from very small and very-very hot to the size and temperature of our current universe. It encircles flusters of star together with several hundred billion other stars in a galaxy soaring through the cosmos. This is the Big Bang Theory.
The Big Bang Theory supports the following evidences
The universe had a beginning and that the universe was once compact.
Galaxies appear to be moving away from us at speeds proportional to their distance according to “hubbles law”, named after Edwin Hubble (1889-1953) who discovered this phenomenon in 1929.
The Big Bang suggests that if the universe was initially very-very hot, it was expected to find some remnant of this heat. It was discovered as 2.725 degree Kelvin Cosmic microwave Background Radiation (CMB) according to Radio astronomers Arno Penzias and Robert Wilson. This is thought to be the remnant of heat.
The abundance of the ‘light elements’ Hydrogen and helium are found of supporting the Big Bang model of origin.
The universe didn’t expand into space, as space didn’t exist before the universe. Instead the big bang simultaneously appears with space everywhere in the universe.
The universe has not expanded from any one spot since the Big Bang rather space itself has been stretching and carrying matter with it.