The universe began with the Big Bang. This was the primal explosion that created energy, matter & space/time. While it is often called an "explosion," it was not an explosion in the ordinary sense. What happened was that space/time began expanding from a point called a singularity and in doing so carried energy and matter with it. Stuff did not explode into space, but space itself was created as the singularity expanded. What happened before the Big Bang or where the singularity came from are questions beyond the scope of science. Science does not concern itself with the existence or nonexistence of God. In a practical sense, the universe was self-creating.
Cosmologists believe the Big Bang occurred 13.8 billion years ago. The universe was hot and dense in the beginning, and energy and matter were interchangeable. Gravity, electromagnetism, & the strong and weak nuclear forces were one.
Evidence for the Big Bang:
1 Edwin Hubble showed the universe to be expanding, that galaxies are moving away from each other. It was easy to conclude that if galaxies are receding, they must have been closer in the past and at one point all together. Expansion of the universe is compared to blowing up a balloon. Dust motes on the balloon are galaxies growing apart.
2 Georges Lemaitre, a Belgian priest, spoke of a "primeval atom." This became the basis for the Big Bang theory.
3 George Gamow predicted that radiation from the Big Bang filled the universe.
4 Arno Penzias and Robert Wilson proved Gamow correct when they detected "cosmic background radiation" using a radio telescope. It is the afterglow from the Big Bang and comes from all directions.
5 The Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP) mapped the cosmic background radiation.
As the universe cooled, inflation took place. The universe grew as energy turned into matter. Protons and neutrons formed. The elements hydrogen and helium came into existence. Heavier particles evolved. As the universe continued to expand, structures appeared. A billion years on, there were stars and galaxies.
Before 1998, it was thought that gravity would slow the expansion of the universe. Observations of supernovas then proved the expansion is accelerating rather than slowing. The force causing the acceleration is called "dark energy." It appears the universe will expand forever.