I brought this up in the TFA thread just before this splintered off into its own discussion, so I’m not sure anyone actually saw it there. With Rakata Prime on the map, I’ve got to wonder if the Rakatans have anything to do with this question. I don’t know if anyone here has read any Alastair Reynolds, but in his books he’s got an ancient race of aliens called the Inhibitors, who essentially booby-trapped the galaxy after an enormous war in the distant past in an attempt to prevent any new space-faring civilizations from developing. I’m thinking maybe the Unknown Regions on the map are what was once the entirety of known space to the Rakatans however many thousands of years before the Republic, and in the dying days of their empire they rigged it as a sort of artificial Bermuda Triangle. If every mission that gets sent in to explore the area ends up lost, I could see the Republic giving up and letting it be.