Nobody knows for sure how the universe began. Over the centuries, however, through observation, mathematics, chemistry, geology, logic, inspiration, and all other human skills, scientists have formulated theories about how the universe began.
The big bang theory is the most widely accepted of these theories. It states that the universe began some 15 or 20 billion years ago in a huge explosion—the big bang. This explosion produced all matter, energy, space, and even time.
Although scientists do not know why or exactly how it happened, they generally agree that this explosion sent the matter hurtling out from its center. As the matter cooled, it formed the particles that make up everything in the universe.
This was the beginning of the universe, the beginning of what would eventually become you and me and everything around us.