It may be defined simply as aeverything that exists. The idea behind the definition certainly is not simple. Imagine that everyhing that exists, from the smallest atomic particle to the largest groups of stars, is part of the universe. Scientists are not sure about the size of the univers.
Some scientists believe that the universe has no boundaries, no edgies, no limits.
This is an idea that is difficult for most of us to understand. Perhaps as scientists continue to make new discoveries, they will be able to better help pus understand just exactly what the size of the universe is. Among them are galaxies, groups of millions to hundreds of billions of stars. Our small, blue planet, Earth is the third planet from the Sun. The Sun and its nine planets make up out solar system.