Flexi Says: According to the Big Bang theory, the Universe began about 13.8 billion years ago. Everything that is now in the Universe was squeezed into a point. It was all in a single, hot, chaotic mass. Then an enormous explosion—a big bang—took place. The big bang caused the Universe to start expanding rapidly. All the matter and energy, even space itself, came out of this explosion. In the first few moments after the Big Bang, the Universe was unimaginably hot and dense. As the Universe expanded, it became less dense. It began to cool. After only a few seconds, protons, neutrons, and electrons could form. After a few minutes, hydrogen came together. There was so much energy that nuclear fusion began. This created helium.