A few days ago I was on my conditioning run, alone for cross country today, I was thinking about cross game connections in bungie games. I then started to think of Destiny and I wondered if Marathon and Destiny were connected. I soon devised a theory of how they are connected, however in an indirect way. Both Marathon and Destiny take place in a similar time frame which makes it very unlikely for both to take place in the same universe. I say it is impossible that they take place in the same universe, but I do say it is conceivable in the same multiverse. (Multiverse defined by a series of universes that are connected by exchange of matter between them)
Now as I previously stated it is impossible for them to be in the same universe, but it is possible to be connected through the multiverse. As we know in the Marathon series the AI Durandal has his ultimate goal set to escape to another universe before the collapse of his own, he wants to "Travel" shall we say. In fact one of his motives is to become godlike, in one of his more famous quotes he says "Escape will make me God". Durandal is also known to have used an "undead"/revived beings in the case of the cyborg in the marathon series in order to do his bidding, we also know that the Traveler uses "undead"/revived beings to do his bidding in the case of the Guardians. I propose that Durandal achieved his goal of escape and ended up in the universe of Destiny, and when he got to the Sol system he caused the golden age for humanity bestowing upon them some of his knowledge.
But what about this ancient enemy the "Darkness"? Well my answer to who they are is that they are the W'rkncacnter which have been called immortal so what if these did the same by changing universes therefore being immortal? They would conceivably rival Durandal in power maybe even so to make a powerful blow against him ending a golden age? (this is my same theory as I wrote on Bungie's forums under my account there as Thespartan274)