Why Does the Universe Expand?

The universe is expanding due to a process called the Big Bang, which is thought to have occurred around 13.8 billion years ago. This expansion is caused by a type of energy known as dark energy, which is believed to make up around 68% of the universe's total mass-energy density. The expansion is accelerating, with the universe doubling in size every 14 billion years. The Big Bang theory suggests that the universe began as a singularity, an infinitely hot and dense point, which expanded rapidly and then continued to expand as it cooled and particles began to form.