• It is also called the expanding universe hypothesis.
• Edwin Hubble, in 1920, provided evidence that the universe is expanding.
• Took place 13.7 billion years ago.
• Galaxies move further and further apart. Similarly, the distance between galaxies is also found to be increasing, thereby indicating that the universe is expanding. 
• Scientists believe that though the space between the galaxies is increasing, observations do not support the expansion of galaxies.
• Stages of Development
  • In the beginning, all matter forming the universe existed in one place in the form of a “tiny ball” (singular atom) with an unimaginably small volume, infinite temperature, and infinite density.
  • At the Big Bang, the “tiny ball” exploded violently, leading to a huge expansion.
  • The expansion continues even to the present day. 
  • As it grew, some energy was converted into matter. 
  • There was particularly rapid expansion within fractions of a second after the bang. 
  • Thereafter, the expansion has slowed down. Within the first three minutes of the Big Bang event, the first atom began to form.
  • Within 300,000 years from the Big Bang, the temperature dropped to 4,500K (Kelvin) and gave rise to atomic matter. 
  • The universe became transparent.

Source: The Hindu