Evidence for the Big Bang
1. Redshift of Galaxies
The light we observe from galaxies has been stretched by the time it reaches us. It looks redder than it should. This redshift is the result of galaxies moving away from us. Observations show that pretty much everything in the Universe is moving apart. The redshift of distant galaxies tells us the Universe is expanding.
If you could wind time backwards, you would see galaxies getting closer together. If you could go back far enough, everything in the Universe would have been in one place.
2. Microwave Background
A long, long time ago, the whole Universe was very hot. As it grew in the size, the heat left a "glow" which fills the entire Universe. The Big Bang theory predicts this glow should still exist. It also predicts that we should be able to detect this glow as microwave light.
Scientist have found this Cosmic Microwave Background. They have accurately measured it using orbiting detectors. It is very good evidence that the Big Bang theory is correct.
3. Mixture of Elements
Some chemical elements were created soon after the Big Bang. Elements like hydrogen and helium. The Big Bang theory predicts how much of each element was made in the early universe. When astronomers look at very old galaxies and stars, the amount of each chemical they see agrees with the Big Bang theory.
You cannot look for this evidence in new stars, like the Sun. This is because newer stars contain chemical elements made by older stars. So the chemical make-up of new stars is very different from stars which existed soon after the Big Bang.
4. Looking back in time
There is an alternative to the Big Bang theory. It is called the Steady State theory. In this theory, the Universe does not change much over time.
Remember - light takes a long time to travel across the Universe. So when we look at galaxies which are very far away, we are also looking back in time.
This means we can see that very old galaxies are very different from newer galaxies. This shows the Universe has changed. This evidence fits better with the Big Bang theory than the Steady State theory.