It shouldn't really be surprising that the Darkness can potentially bend reality; look at what it's "ancient enemy" the Traveler can do.
The other races make me curious though. My theory is that the Fallen survived an encounter with the Darkness and now chase it across the stars to scavenge the civilizations it destroys. The Cabal aren't connected to at the Darkness all; they're just expansionist Space Romans who see Sol as their next conquest. The Hive are clearly creations/worshipers of the Darkness, and the only enemy race with an obvious direct connection. The Vex are still wrapped in mystery.
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My personal theory for the Vex (I have no basis for this, only a suspicion) - They are the last remnants of humanity, or a creation of humanity, sent back in time to fight the Darkness, perhaps because the Guardians failed.
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In a video doc(I forget which) Bungie said that the Vex were time traveling robots. This could mean that they are coming back to either try to stop the darkness or help it. But again this is all speculation.
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Awhile back i thought the vex might be of human creation or discovery just as a guess but had no real evidence to support it. :/
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I thought vex are like the geth where they might be split. Ones who came to harm, and possibly others to warn of the future. Heh they might be coming through time to find supplies to fight the darkness themselves. Theories