Step-by-step explanation:
Big Bang theory is the most widely approved theory of Universe creation. According to this theory the Universe is around 13.8 billion years old. Before that everything was concentrated in very high temperature and highly dense state. The universe expanded (not exploded as commonly known) from this state. The two major evidence to support this theory:
1. The galaxies are moving away from us and each other. Also, the galaxies which are farther from us are moving at a faster rate than a galaxy nearby.
2. The discovery of Cosmic Microwave Background (CMB) radiation in 1964 further proved this theory correct. The presence of CMB could be proved only by Big Bang model and not by Steady State model of the universe.