As far as we know there is no material that stretches or expands, it is simply that distances between remote galaxies increase by a certain percentage each year.
Galaxies close together can be held in a cluster by gravity, orbiting each other as a group.
Only the very large distances take part in this pattern of increase.
It is a small percentage. Amounting only about 1/140 of a percent every million years.
The pattern of increasing distances is called Hubble Law. There is no limit on the amount distances can increase, according to that law.
The balloon analogy is mean to help people learn to imagine the pattern of increasing distances. It is not a physical or material model. Empty space is not like rubber or anything. check out the animations of the balloon analogy to get in mind how distances between stationary galaxies increase, and how this affects the distances that photons of light can travel over time (they are shown as wrigglers in the computer animation).