The universe is space and everything in it. It is made of many billions of stars and planets and enormous clouds of gas separated by big spaces. The Big Bang started the expansion of the universe.

Astronomers use telescopes to look at distant galaxies. This is how they see what the universe looked like a long time ago. The past tense is because the light from distant parts of the Universe takes a very long time to reach us. From these observations, it seems the physical laws and constants of the universe have not changed.