The doppler effect, when used with light, produces a shift of frequencies to red. Well, no matter which direction you look, there is a shift to red! That means that the universe is expanding or moving away from us in every direction. The idea, is that this could only happen if all matter was once in a single point, and there was an explosion. This explosion from a single point is called the big bang theory.
A nebula is a cloud of gases that can coagulate to become a new star.
