I'm pretty sure the concept of the Unknown Region still exists in the ST era. That's where Palpatine was hiding after his essence transfer, Exegol.

There is also the galactic core that is so densely packed with stars that hyperspace travel is incredibly dangerous. So I think maybe there's a lot of undiscovered systems in there.

I am not sure

We know space is really massive and many parts of Space in real life haven’t been explored so it could be the same in Star Wars

Coruscant is a Core world they mentioned that this galaxy has a supermassive black hole in the center.

Galaxies in general are very dangerous to travel to the center massive black Holes reside there our own Milky Way has one as well.

The planet mentioned that is dangerous to travel is Typhon due to how deep in the center, and the hyperspace lanes.