问答题 Astronomers believe that the expanding universe is the result of an enormous and powerful explosion called the Big Bang. The Big Bang theory may explain how the universe formed. The Big Bang theory states that the universe began to expand with the explosion of concentrated matter and energy and has been expanding ever since. According to the theory, all the matter and energy in the universe was once concentrated into a single place. This place, of course, was extremely hot and dense. Then some 15 to 20 billion years ago, an explosion -- the Big Bang -- shot the concentrated matter and energy in all directions. The fastest moving matter traveled farthest away. Energy, too, began moving away from the area of the Big Bang.