If itís hard to grasp the size of the universe, itís equally hard to imagine its age. Astronomers calculate that the universe originated about 13 billion years ago in an explosion of space called the big bang. At that time, all the matter and energy in our observable universe was packed together in a volume smaller than an atom.
No one knows what happened before the big bang or what caused the explosionóour laws of physics canít explain itóbut in an instant the energy and matter of our universe poured into existence and expanded with space itself. We still see the evidence of the big bang today, as the superclusters of galaxies continue to fly apart from each other. We also observe the remnant glow of the big bang itself in the form of faint microwave light from all parts of the sky.
The early universe was small and intensely hot. As the universe expanded and cooled, small particles condensed and formed the elements hydrogen and helium. Over billions of years gravity allowed the cooling gases to collapse into galaxies, stars, and planets, including the Earth. Here's a timeline of important events in the history of the Universe. The timeline is in billions of years, so we need a magnifying glass to show when events occurred only millions of years ago.
For a truly unique perspective on the history of the Universe, check out the "Universe in One Year." This shows the past 13 billion years compressed into a single year!