Origin of the Universe

The majority of scientists believe the universe was created in a 'Big Bang' - the universe was formed from a singularity and the universe has been expanding since. The theory was developed by Georges Lemaître in the 1927. A rival theory was widely discussed called the 'Steady State' theory which considers the universe has no finite beginning or end.

Steady State Theory

Big Bang Theory

These are not the only scientific theories, others exist and are worth looking at.

Inflationary Universe Theory: Inflation cannot be stopped, new universes that we are not yet aware of are being created. We are just one of these.
Cyclic / Oscillating Universe Theory: A big bang, then a colossal implosion (a big crunch) which then starts another big bang and the cycle repeats.