I don't know if you remember or will see this but you're forgetting about the Jovian's and the golden age part. We colonized moons of jovian planets, which fell first to the darkness. Xur is of the Jovian's, somehow able to travel between the jovian (gas) planets and earth. It is said that "the light" hurts them, also that there are 9 levithian intelligences that now rule over those planets. It is assumed that they are descendants of those humans, much like how awoken are humans transformed. Just something to maybe add