Post 2 addresses the observable universe. As far as is known every observer from every galaxy also is emeshed in a similar observable universe...which may or may not overlap with ours. Big bang and inflation theory suggests we can see only a tiny,tiny part of our own universe and as noted there might be an infinite number of universes...another might start right at your nose, for example, and be undetectable. This can also be hypothesized via quantum and string theories.
Brian Greene's FABRIC OF THE COSMOS is one non mathematical book which gives good insights to these different explanations.
It's also interesting to note that space also ends inside a black hole where it is replaced by time...?? even future and past!. This is analogous to an extreme case case of relativity where two observers in relative motion each see the other a mix of space and time (if we are in relative motion, my space is a mix of your space and your time, and your space is a mix of my space and my time).
In other words, in a sense, space might meld into time and vice versa.
Have you ever wondered how small space is? Is it continuous or discrete? In a way that might be considered analogous to a black hole, at tiny, tiny scales (planck scale) it appears that space,time,energy lose their individuality...they are apparently not identifiable but do they decompose or instead maybe meld into a "quantum foam" of uniformity...
