Although How the Big Bang Theory Works covers the origin of the universe in detail, it will be useful to cover the basics here. The short version goes like this: About 15 billion years ago, all matter and energy was bottled up in an incredibly small region known as a singularity. In an instant, this single point of super-dense material began to expand at an astonishingly rapid rate. Astronomers don't fully understand what caused the expansion to begin, but they use the term "big bang" to describe both the singularity and the first few moments that followed.
As the newborn universe expanded, it began to cool down and become less dense. Think of a jet of steam issuing from a teakettle. Near the spout lid, the steam is quite hot, and the steam molecules are concentrated in a confined space. As the steam moves away from the kettle, however, the steam cools down as the molecules spread throughout your kitchen. The same thing happened after the big bang. Within roughly 300,000 years, everything held within the singularity had expanded into a seething, opaque sphere of matter and radiation. As it did, the temperature dropped to 5,432 degrees Fahrenheit (3,000 degrees Celsius), allowing more stable particles to form. First came electrons and protons, which then combined to form hydrogen and helium atoms.
The universe continued to expand and thin out. You might be tempted to picture this young universe as a stew, with clumps of matter floating in thick gravy. But astronomers now think it was more like a soup, very smooth in density except for a few tiny fluctuations. These disturbances were just significant enough to cause matter to coalesce. Huge clusters of protogalaxies began to form. The protogalaxies matured into galaxies, great islands of gas and dust that gave birth to billions of stars. Around some of those stars, gravity pulled together rocks, ice and other materials to form planets. On at least one of those planets, life evolved, some 11 billion years after the big bang started it all.
Today, the universe continues to expand, and astronomers have evidence to prove it. Up next, we're going to examine some of that evidence.