The Dark Age was a period of time that followed The Collapse. It was an era of a Thousand Kings. There were just as many torments and much suffering.
It was during this era that the Awoken appeared and the Exos came out of the ruins. The Exos had lost their memories so why they were made remains unknown.
Toward the end of this era the Ghosts first appeared. They began hunting for people who could become Guardians. This is when the rumors began that the Traveler would become The City one day.