Scientists have proposed two main theories for the origin of the Universe. Both agree that the Universe is expanding, so that galaxies are moving apart. They disagree about whether the Universe had a beginning.
The Big Bang theory says that the Universe began about 13.8 billion years ago from a very small, very hot and very dense point. Space has been expanding ever since, and the Universe has changed over time. In the past it was much hotter and denser than it is now.
The Steady State theory says that the Universe has no beginning and no end. On a large scale it always looks the same and its average density never changes. As the galaxies move apart, new matter is continuously created in the gaps between them, which keeps the density constant.
