Illustration of the expansion of the Universe. The Cosmos began 13.7 billion years ago in an event dubbed the Big Bang (left). Immediately it began expanding and cooling (stage 1). Eventually, the universe became transparent to radiation, and the first matter was able to form into clumps. Its expansion slowed about 10 billion years ago (stage 2). At stage 3, 5 billion years ago, the universe was full of stars and galaxies, and its expansion began to speed up again because of the mysterious Dark Energy that pervades the Universe.
