An astronomer named Georges Lemaître proposed that a very long time ago, the universe started as just a single point. He said the universe stretched and expanded to get as big as it is now, and that it could keep on stretching.When the universe began, it was just hot, tiny particles mixed with light and energy. It was nothing like what we see now. As everything expanded and took up more space, it cooled down. This theory came to be called as The Big Bang Theory. The summarised points of this theory are as follows-

1. The tiny particles grouped together. They formed atoms. Then those atoms grouped together. These atoms came together to form stars and galaxies.

2. The first stars created bigger atoms and groups of atoms called molecules. That led to more stars being born.

3. At the same time, galaxies were crashing and grouping together. As new stars were being born and dying, then things like asteroids, comets, planets, and black holes formed.