What is beyond the limits is outside of the universe and perception. It may be truly nothing. Maybe it’s everything. But what ever it is, it impossible to know from here. Here we be finite.
I read a decent bit of SciFi, and then forget most of it. One of the few really memorable pieces I’ve read was Tau Zero by Poul Anderson. Based on your posts, if you haven’t read it, I’d recommend you check it out.