I think you should abandon this "vacuum" theory.

Pressure is a measure of the average kinetic energy of particles hitting a surface.

Now, even if that "outside" space existed, galaxies do not collide continuously and try to "enlarge" the space they are in. So a galaxy in this hypothetical universe boundary will not be pushed toward that outer space from other galaxies hitting the galaxy.

In addition, probably there is not that "outer space" emptier than the inner space. As far as I know, no theory make hypothesis on a "emptier outer space".

From what I read from Ethan (and others), what we know is:

- The age of our observable universe is about 13,8 billions years

- The universe radius is more than 46.5 billions years

- The observable universe radius about 46.5 billions years

- The universe space could be flat (which data suggest for now), curved or curved closing.

If the universe is curved closed there is no other outer space, no boundaries, no center.

Also if it is curved close is much, much bigger than 46.5 billions light years.

Even if it is not curved closing, the space/time fabric could derive from other fundamental properties of quantum theories.

I don't have enough good knowledge to write about the universe boundaries, but it is likely that there is no "outer" universe space.

One personal consideration, your positive attitude is, in my opinion, very useful to understand what we can about physics. I make your same "mistakes", because understanding something makes you always postulate some theories. The problem is that we cannot (at least it is valid for me) understand the mathematics behind theories, and most of the paradoxes are due to a poor description of mathematical concepts.