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. Physicists are currently unsure if anything existed before the Big Bang. The size of the universe is not known. History People have long had ideas about the universe. They saw the sky at night, with fixed stars and other stars moving among them. Most early ideas had the Earth at the centre of the universe. This is known as geocentrism. Some ancient Greeks thought that the universe has infinite space and has existed forever. They thought it had a set of celestial spheres which corresponded to the fixed stars, the Sun and various planets. The spheres circled about a round but unmoving Earth. ...