Caption: The expansion of the universe is general, uniform scaling up of gravitationally unbound systems ---it's everywhere---everywhere we can see that is: i.e., the observable universe. There is NO center of expansion and nothing is being expanded into as far as we can see in the observable universe.
Features:
General relativity (GR) along with other assumptions predicts the expansion of the universe. GR tells us the expansion is a literal growth of space---space is a kind of stuff/structure that can grow or shrink.
The comoving frames, in fact, form a continuum of fundamental free-fall frames throughout the observable universe.
He hypothesized that the reference frame in which the fixed stars were on average at rest was the singular fundamental inertial frame which he called absolute space. All other exact inertial frames were unaccelerated relative to absolute space.
This means we can then calculate the internal motions of galaxy clusters and field galaxies using Newtonian physics for the most part.
For the observable universe as whole, we are beyond the realm of validity of using a single inertial frame and must use general relativity in a more direct sense.
Well it may be then that the universe is infinite and getting bigger.
Or it may be that the universe is a finite hypersphere---a 4-dimensional sphere with a curved 3-dimensional surface, but with the curvature so small we have NOT detected it---we may be microbes on a billard ball.
We do NOT know, but there are many theories.
The multiverse paradigm is one huge cluster of such theories. The usual idea is that there is a background false-vacuum universe full of pocket universes in which different conditions and even different physical laws hold. The observable universe is embedded so deeply in our pocket universe that we do NOT see any trace of its boundaries.