So I know there were some hints and stuff to Sins history in the past but there seem to be some interesting ones in this latest DLC. For instance one thing I found quite interesting was the description for "ancient mine factory"- the description states something along the lines of "An abandoned mine factory is found, containing elements of both modern TEC and modern Advent architecture, yet is clearly neither. Increases maximum space mines in this gravity well by 25." This to me suggests that the Advent and TEC were brought together in the distant past by some unknown third force, possibly along with other factions. Presumably this was the same race that left behind the relics we all enjoy finding so much. Whether or not they were brought together willingly or not is the question .
There's also ablative crust, which states that the planet bears pockmarks from a great, devastating battle in the distant past, and that the ablative crust was the only reason the planet wasn't shattered. This is a pretty obvious hint that there was some devastating war in the past that presumably wiped out the race that left the artifacts behind. I'd write more but my train of thought is being interrupted by a Skype convo.