1. The Universe Today & The Big Bang

2. The Big Bang Astronomers generally agree that the universe began between 10 and 20 billion years ago. As the universe appears to be expanding in all directions, it is thought that at one point the area of the universe must have been zero, or at least infinitely small. It is not understood what caused the universe to start expanding, as no one knows what the laws of physics would have been at this point.

3. After The Big Bang Early on, when the universe was still very small, events happened very quickly. It was so hot, only small particles such as protons, electrons and neutrons could exist. Atoms of hydrogen only began to form when the universe was sufficiently cold - 500,000 years later.

4. The Universe Today This picture shows what the universe looks like today. The blue bits are galaxy clusters. It is very big. The observable universe is the Universe that reveals itself through electromagnetic radiation that can be detected on Earth.