The Big Bang
It all began 13.8 billion years ago. The entire universe which spreads billions of light years today, was once a size of electron which exploded to about it's current size within a fraction of second. This came to be famously known as The Big Bang. Even the time did not exist before the big bang. English astronomer Fred Hoyle first coined the term "Big Bang". A Belgian priest George Lemaitre first suggested the Big bang theory in the 1920s.
Big Bang on a time scale
After its initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, and later atoms. Giant clouds of the primordial elements were formed. These atoms combined to form larger atoms and elements. The earliest of them were hydrogen and helium. The strong forces of gravity led to the formation of gas clouds. Due to the gravity, hydrogen and helium atoms started combining which gave rise to the higher elements. According to the Big Bang theory, the universe at the beginning was very hot and very small, and since then it has been expanding and cooling down. From the day big bang took place, the universe is continuously expanding.
But the question is,
" How valid is this theory..?? "
" How come the scientists are so sure about this theory..?? "
There are many questions which ask the validity of Big bang theory, but many experiments and results obtained synchronizes exactly with what the Big bang theory predicts, thus making it true.
Results obtained from the astronomical observations, data from the computer simulations and experiments from the Large Hadron collider predicts that Big bang has actually happened. Where as it's proof is present in the form of Cosmic microwave background
Large Hadron Collider, CERN
Big bang even predicts that, the size of Universe was of size of a sand grain when it's age was negligibly small and it expanded billion billion times of it's initial size within a fraction of second.
The Universe which is about 13.8 billion light years has got enormous number of stars and planets. Only a small part of these Universe is visible for us, which we call The Observable Universe. In this observable universe, we can find cosmic web, which has trillions of local group of galaxy cluster.