The Golden Age was an era of Prosperity. It was brought on when the Traveler appeared on Earth. This was a centuries long age that helped humanity colonize the whole of the Solar System. It was during this time that the Exos were created.
The Traveler taught humanity much. This gave humanity a number of new powers and they gained access to mystical technologies. It was using this that Humanity spread itself out among the Solar System that it inhabited. Every planet felt the touch of them. This expanded the technolgies even further.