Our current understanding of the universe is based on the Big Bang model. The model proposes that approximately 13.8 billion years ago, the universe came into existence as a singularity and rapidly expanded from an extremely hot dense state to what exists today. It is still unclear why this happened or what a singularity actually is, but we do know that the universe is still expanding at an increasing rate.
If the start of the universe was so hot, where has all that energy gone? The Big Bang theory predicts that as the universe expanded, it cooled. Cosmic Microwave Background Radiation has been detected in space as evidence of this cooling. The average temperature of the universe is currently around \(2.7 K\).
Observations of distant stars, show what is called a red shift in their colour spectrum. This red shift in galaxies is evidence the galaxies are moving away from us, indicating that they are moving further away from a single point when the Big Bang occurred.