rumor is the Vex are servants of the traveler and the traveler feeds on civilizations through space magic. when it is time to move one the vex come and wipe out life in the universe. somehow, its plans failed when it reached humanity in this time line and its trying to figure out how to wipe us out and get away. this is why Rasputin crippled the traveler, to prevent the end of humanity by the travelers hands.