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?
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My own personal theory they where once humans who gave up their fleshy useless bodies for the greatness that is the machine! Of course they F'ed up the process and lost their memoires. Or they lost a war and the memoires where wiped.