Board Thread:Destiny Forum/@comment-5820164-20130403210343/@comment-7622429-20140614212529

Well, to me, the key idea is that the main struggle is between the Darkness and Humanity. This means everything about our enemies revolves around this core idea. The Fallen and Cabal are likely just glorified mercenaries, the Hive are probably an enthralled race personally belonging to the Darkness, and then we have the Vex. They present the most unknown variables as to their connection to the Darkness.

Personally, I've always liked to think that they're the more advanced biomechanical army of the Darkness, somewhat the complete opposite of the Hive, whom are very organic and wield dark magical powers. The time-traveling part means that determining when they came from is a challenge. However, I'd like to think that the Vex are a machine army manufactured by the Darkness in its more glorious times, likely sent from the past to aid in the modern Darkness' plans.

However, this is all just speculation on my part, so take it with a grain of salt.