Which of the following statements best describes the Big Bang Theory? The Big Bang Theory describes the formation of the universe, which scientists believe happened 13.7 billion years ago. Under this theory, the universe began as a very hot, very dense point in space that began expanding outward. It still expands today. The Big Bang Theory describes the formation of the solar system. The solar system was formed 13.7 billion years ago. It is still expanding today. The Big Bang Theory describes the formation of the sun. The sun began as a very hot, very dense point in space that began expanding outward. It still expands today. The Big Bang Theory describes the formation of the universe, which scientists believe happened 13.7 billion years ago. Under this theory, the universe began as a very hot, very dense point in space that exploded. It stopped expanding 1 billion years ago.