I don't trust the aliens.
But Exo I could see being a group of people who have tended towards technological integration to the point of sacrificing their humanity. They were heading towards singularity during the Golden Age, but are now relics of the past. Unable to die, but unable to progress because of technological regression.
Awoken... an appeal towards the ladies in the Bungie community. Before you flame me, c'mon, they had a picture of Edward Cullen on their inspiration page.
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I really like the Exo explanation. Makes a lot of sense actually.