The universe is the name that we use to describe the collection of all the things that exist in space. It is made of many millions of millions of stars and planets and enormous clouds of gas separated by a gigantic empty space which is called the universe.
Astronomers can use telescopes to look at very distant galaxies. This is how they see what the universe looked like a long time ago. This 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.
Many people in history had ideas to explain the universe. Most early models had the Earth at the centre of the Universe. Some ancient Greeks thought that the Universe has infinite space and has existed forever. They thought it had a set of spheres which corresponded to the fixed stars, the Sun and various planets. The spheres circled about a spherical but unmoving Earth.