What is big bang theory

The Big Bang theory also know know as the "expanding universe theory " is one of the most important theories explaining origin of the universe. It was Edwin Hubble, who provided evidence regarding the expanding nature of the universe. The expansion occurs in such a way that the distance between the stars and galaxies increases without any increase in the size of the stars. With the benefit of improved telescopes, Hubble proved that the light coming from these galaxies was shifted a little towards the red end of the spectrum due to the Doppler effect (known as “redshift”), which indicated that the galaxies were moving away from us.
Inflated balloon theory as a simple way to demonstrate the expanding universe theory is contradictory in a way that the size of the dots in an inflating balloon increases but that's not true for the size of the stars.

The explanation of the origin of the universe can be summarized in the following steps :
1.  In the beginning all matter was consolidated into a single tiny ball(singular atom ) with zero volume, infinite mass and temperature.
2. When the Big Bang took place around 13.7 billion years ago it resulted in explosion of the tiny ball and the formation of the universe. The expansion of the universe is still taking place but at a slower rate.
3.After 300,000 years the temperature of the universe decreased to 4500 K which resulted in the formation of atomic matter. There have been several theories regarding the origin of the universe but the Big Bang theory is the most widely accepted one.