This is the canon map before the Disneyfication, so its the map we will likely be sticking with (why would we need to be purely'new canon', no fun there.) As you can see theres tons of empty space, I do not know if there are any plans to fill it out.
Because something can or is done in Darkness does not mean it is done in RP, however, despite what many seem to belief. For instance, hyperspace in Darkness is a 'warp' function similar to Star Trek, whereas in reality people travel along widely established hyperspace lanes. Scouting the galaxy is extremely slow and extremely dangerous. We're talking a process of years just to find one new system, and that's at the fastest.
The Western part of the Galaxy has always been known as the 'Unknown Regions', a place of myth and danger that nobody in their right mind ever went into. It's just kind of a staple of the Star Wars setting. I don't see us exploring it to any meaningful degree anytime soon.
"Some hypotheses over the hyperspace turbulence in the Unknown Regions claimed that it was created to serve as a barrier between the Celestials and the upstart Rakata. Despite the Celestials' efforts, the Rakata broke through the barrier and waged a war of extermination upon them."
It's not really known if the 'barrier' was intentional or the result of some form of accident, but yes, this barrier made it nigh-impossible to travel properly in western space. This, in addition to the inherent difficulties in scouting out new space, essentially meant there was no progress on that front since nobody in their right mind did it.
I don't think it was a barrier that stopped people. It was highly unpredictable and not charted, so the danger of Hypering through there was extreme. I elieve Outbound Flight followed a previously scouted path that was deemed safe.