The universe is made up of everything that is found anywhere in space. It includes Earth, the sun, all the other planets and stars, and everything on them and between them. Scientists who study the universe are called astronomers. Image to left:The universe includes Earth, the sun, all the other planets and stars, and everything on them and between them. No one knows how big the universe is. Credit: World Book illustration by Ernest Norcia No one knows how big the universe is. Today, astronomers believe that the universe is expanding, or getting bigger. They think that every part of the universe is moving away from every other part. But they do not know if the universe will expand forever or if it will someday start to shrink.

Nearly all astronomers believe that the universe began between 10 billion and 20 billion years ago with an explosion called the big bang. According to the big bang theory, the universe has been expanding since it began. At first, the universe was mostly just energy. But as the universe got bigger, most of this energy changed into gases. Some gases later grouped together to form stars, planets, and other objects in space.

Astronomers use powerful telescopes both on Earth and in space to study the universe. Such telescopes help them see objects very far away. Computers are also helpful for astronomers. They control telescopes and other tools. Astronomers also use computers to work out information collected with telescopes. How to cite this article: To cite this article, World Book recommends the following format: "Universe." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.