What really happened during Big Bang?

BIG BANG 

The big bang is not actually a bang rather it is a sudden expansion of the point of dense mass, energy, and spacetime (whole universe). The singularity itself expanded and created different particles, energy, forces, and also spacetime; the whole universe we see today.

 The big bang theory was proposed by Le Maitre and Gamow. The idea of expanding the universe leads to the idea that before in time all the galaxies were closer to each other as compared to their current position. This is the fact applied in the big bang. According to big bang theory whole of this universe was concentrated in a highly dense and hot singularity having a temperature of about 1012 K. About 19.8 billion years ago a huge explosion took place and the singularity (a single point of highly dense and extremely hot fireball) burst out and was thrown out with extremely high speed in all directions forming stars and galaxies. All these are galaxies and stars are still moving away from each other.