Evolution — BIOLOGY STD 12 Science — Question

The universe is about 15-20 billion years old. It comprises of huge clusters of galaxies, which contains stars, clouds of gas and dust. According to the Big Bang theory given by Abbe Lemaitre (1931), our universe arose as cataclysmic eruption. The temperature lowered and the gases like hydrogen and helium were formed sometimes later. By the force of gravitation, the gases condensed and galaxies were formed.