The Big Bang
The Universe started out about 13.7 billion years ago with the Big Bang. This is the theory that the Universe started from an explosion and then began expanding from its very dense and hot state. Georges Lemaître originally proposed this theory in 1927, and it has been backed up by the works of Edwin Hubble and other astronomers. As the Universe expanded it began to cool from its extreme temperatures, allowing matter to form. Then gravity began forming the very first stars and galaxies from clouds of gas. All the matter that exists in the Universe today came from this explosion and was once tightly compacted, but now after the Universe has been continuously expanding for billions of years, it has spread apart vastly around the Universe. According to Las Cumbres Observatory, "As the universe expanded, pockets of gas became more dense and stars began to ignite. Groups of these stars became early galaxies." At about 3 billion years after the Big Bang, smaller galaxies began merging to create much larger galaxies, these collisions were so brutal that massive gas clouds and stars collapsed to form black holes (The Early Universe, n.d.). About 12.6 billion years ago our very own Milky Way Galaxy began to form, but it wouldn't be till billions of years later that it stabilizes enough to allow the formation of the Sun and our Solar System.