The Big Bang is the most widely accepted theory on how the Universe was created. Many years ago, a lot of astronomers thought that the Universe had no beginning or end. They believed it was infinite. But This theory proved that the Universe had a beginning and it isn't infinite.

This huge explosion happened about 15 billion years ago. The minute this huge explosion occured , the Universe started heavily expanding. Even to this day. During the time of this event, all the matter and energy in the Universe was contained in one point. As of now, scientists and astronomers don't know what existed, prior to the Big Bang. Many people just describe this as "nothing". It's often questioned, what was this "nothing". But obviously, no one actually knows what there was.

Ever this has happened, the Universe has been expanding constantly. The main person behind the idea of this theory is Edwin Hubble. He's highly credited for making the observation of the Universe expanding. Since the Universe is always expanding, the distance between galaxies is always increasing. Which means, that there was a time when all the galaxies were tightly packed together into a single point. And the distance between them gradually increased.

Although, This is the most widely accepted theory on how the Universe was created. There are still many people that don't believe in The Big Bang. There are various other theories out there, that explain how the Universe was created. But again, this one is the most widely accepted.