Stages in the history of the Universe after the Big Bang. At 300,000 years, matter and radiation decouple, and the Universe becomes transparent for the first time. The first galaxies form at around 300 million years after the Big Bang, but the exact time period is uncertain. At an age of 9 billion years the Solar System formed. Finally, 13.7 billion years after the Big Bang we arrive at the present day. The Universe is still expanding and evolving.