The rubber of the balloon skin represents the entire knowable universe in all its multiple dimensions.
Imagine (if you will) a balloon made of very thick rubber. This rubber has been printed or doped with a special technique so that it has pictures of stars and galaxies within it.
When the balloon is blown up: imagine that not only the balloon expands, but the thick rubber skin expands ‘sideways’.
That’s the model. Space expanding in all directions and everything IN space - the stars and galaxies - getting further apart.
But don’t think about the space inside the balloon. That isn’t part of the analogy anymore than the air-pump blowing up the balloon is.
Hope this is helpful.