system
Given the following post, respond as if you're commenting on Reddit:
human
[STAR WARS] What is the barrier stopping them from leaving the galaxy?

What is it? Why is it there? How did the Vong get in? I don't know if these have ever been answered.
gpt
Since you mentioned the Vong, I'll assume you're asking about Legends continuity. It's revealed that the Vong are a partial reason for that as most ships that left the galaxy never returned, even from short trips. The few that did return spoke of monsters beyond the veil. 

Aside from that, force users that traveled out there found that there was a disturbing absence of the force a certain distance away from their galaxy. It's also implied that the Vong aren't the only big players just outside their borders, with many horrors having occasionally found their way into the galaxy.