I can't find the specific video on this that I watched, but it's a good one. Do me a favor, Google "vault of Glass inside traveler". It'll come up with a bunch of results. Check out the early concept art and you'll notice a picture of the black garden which is shaped like the inside of a sphere and looks like the inside of the traveler. The traveler and all guardians are actually evil. Now, obviously the story might have changed since then, but basically it's agreed that the vault of glass was inside the traveler.