Hopefully in the story, or sidequest, or a dlc I don't care which we get to find out who built the exo. Maybe a counter force anpgainst the Vex? A weaponized ghost companion? Souls of those returned from the void?
Exos were built by the Humans during the Golden Age. For what purpose, we don't quite know, though they are described as tireless war machines. Hopefully we get more information on their background. I'm writing a story about the end of the Golden Age that puts in my own thought and ideas. I'll post it here in #Destiny when it's done.