The universe started in an event known as the Big Bang,
which occurred about 13.7 billion years ago. It was a type of
explosion that produced everything in today’s universe—all
energy, matter, and space —and marked the start of time.
Back then, the universe looked nothing like it does today,
but everything that exists now existed in some form then.
Although the amount of material and energy the universe
is made of has remained the same, it has been cooling,
expanding, and changing ever since it came into being.