I'm not sure if there is any explanation about the Darkness that is chasing the Traveler so far, but I have an idea about how the Darkness came to be. I think the Traveler was looking for a perfect race to uplift - one that can spread life to the rest of the galaxy. We are that perfect race, which is why the Traveler decided to stay and defend us from the Darkness. As we have seen so far, multiple races are part of the Darkness, including the Hive and the Fallen. I think the Traveler visited the homeworlds of the Hive and Fallen, seeing if they had the capability to be a perfect species. The Fallen was too brutal a race to work, and as far as I can tell, not a single being in the Hive has free will, possibly relying on a queen, similar to insect species. When the Traveler noticed their brutality and inability to be a perfect race, it left to find a new species to attempt to uplift - humans. As with us, the Fallen and Hive has based a religion around the Traveler. When it left, they decided to hunt it down and destroy it for betraying them. When they discovered that the Traveler had uplifted us and has seen us as a perfect candidate, the previous candidates wanted to destroy humanity. That's what caused the end of the Golden Age. What do you think the Darkness came from?