The Unknown Regions are a horrific mess of Gravitic anomalies, super-dense stars, high energy pulsars and other massive stellar obstructions to Hyperspace Travel. So they are called that because it's pretty much Space Antarctica and is incredibly difficult to explore.
And Jocasta Nu's comment has been considered incredibly moronic for years. The Yuuzhan Vong exist and they aren't in the records as are a great, great many things.