I was inspired by the speculation in the comments on my last theory to elaborate upon the strange relationship the Vex and the Hive have with the Darkness. The Fallen are pirates and scavengers, albeit with a rather confrontational attitude about it, and the Cabal seem to have their own agenda, but the other two seem to be directly working for or as the Darkness itself. The Hive, as evidenced by Grimoire Card entries (thanks to Fez PY for bring that to my attention) use weapons and powers that are actually GIVEN to them by the Dark. Shredders have no firing mechanisms, Wizards pull powers from the Darkness much like Guardians draw strength from the Traveller's Light. The Vex, however, are not as gifted. The Vex's weapons do pull their ammunition from somewhere/when, in a similar fashion to the Hive, but the Vex reek of anti-theism; Atheon's name is derived from atheist, or godlessness, and they seem to replace the Hive's religious fanaticism with transcendent technology. "But that makes no sense! The Vex were WORSHIPPING the Heart of the Black Garden and their best fighters are named Zealots, surely this points to them worshipping the Darkness?" I thought. Then, I hit upon a new idea: The Vex don't worship the Darkness, they copy it and seek to replace it. Its a radical idea, but the Exo Stranger's words "Darkness so evil it hates other Darkness" stuck with me. The Vex worship themselves, they seek to become Gods. For all we know, the Vex built the Black Garden (they seem to be the only race there or trying to get there) and the Heart might be their own creation. This wouldn't be their first, or last, attempt to build a God; Atheon's Grimoire states that the Vex wish to worm into the very fabric of creation, much in the way whatever rules the Hive seems to. They're close as well, manipulation of time has given them similar abilities. Ultimately, though, these questions can never be answered until we visit a planet or moon where both the Vex and Hive meet (Mercury, anyone?) and observe their interactions. If they fought, it might justify my theory that the Hive were chosen by the Darkness and are gifted their strength that the Vex now seek to emulate. The idea that the Vex seek godlike powers is hinted in the Grimoire cards of the Templar and, if interpreted that the Garden's Heart is a Vex creation, the Sol Progeny as well. My final piece of evidence is located in Crota's own Grimoire Card written by Ikora (a greater font of information I know not) in which, at the very end, she links Hive Netherworlds to the Vex Gate system. The Hive and their worlds are created by the Darkness who seem to have chosen the Hive as their champions, the Vex's trans temporal dimensions seem to be of their own design. In conclusion, I bring us back to the words of the Exo Stranger "Darkness so evil it hates other Darkness". Notice how her greatest fear is the Vex, not the Hive who are in a prime location to invade Earth. Perhaps in her future, the Vex become their own Dark Gods. If so, the future bodes ill for both the Light and the Dark.