The Big Bang theory is a model used to explain the:
The Big Bang theory is the most popular and widely accepted explanation for how the universe began. According to the Big Bang model, all of the current and past matter in the universe came into existence at the same time (about 13.8 billion years ago). At this time, all matter was compacted into a very small ball with infinite density called a singularity. Suddenly, the singularity expanded, and the universe as we know it began.