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?
From what I recall in the grimoire, an earlier commenter is correct, exos were crafted by humans in a last ditch effort to protect themselves from the darkness. But clearly that didn't go too well.