How do we know the big bang happened if there was no-one there to see it?

As you know, the big bang is how the universe first began. It began from a single point that expanded in a giant burst/explosion of matter and energy. The first evidence this happened is that the universe is still expanding. All the galaxies in the universe are moving away from each other. Imagine that these galaxies are drawn onto the surface of a balloon and that balloon is being blown up. Working backwards - shrinking your balloon - they must have come from one point. The real evidence for the big bang is that the universe is filled with very weak radio waves. This is the remains of the very first light the universe ever emitted and its called cosmic background radiation (CBR). Think about the early universe as a "soup" of particles that were so dense and so close together, that light kept bouncing off them and couldn't escape. But as this mass of particles expanded, they became less dense until they reached a point at which light could escape. So the universe switched on like a lightbulb. And we can see the evidence of that "first light" today! CBR was first detected by a couple of electrical engineers - Penzias and Wilson - in 1965. They were just doing radio measurements and they thought they had a faulty piece of kit ! But they'd just detected the birth of the universe! It's not every day you do that. So there you are - we know that the big bang happened. But no-one knows why it happened!