Our intuitive sensory anticipation is that if space is finite but endlessly expanding there must be a border. But there isn't, at least not one we can reach. Of if you like we are always at it.
Consider 2-dimensional people living on the balloon. The "border" is up or "down" but they have no intuitive notion of up or down. If they move around on the balloon and get back to where they came, it will appear to them that they went in a circle.
Space is "bent" in a way if you like, but not within our dimensions. And the amount of bending is different where there is gravity.
Thinking classically and using your balloon example, as space-time is expanding it is filling the nothingness — the nothingness that existed before the big bang?
The Big Bang model does not tell us of anything "outside" of it or "before" it. Now logically, since our universe seems to be made up of derivative things and any subset of all the things in this universe in aggregate are derivative, even as you get close to and include the entire universe in the subset, then the universe does not have the self existent quality that is called "Aseity" by those studying metaphysics and theology seriously. That is, the property where something itself does not need a "cause" and/or does not derive its existence from something external.
This is a subject, I have gone very deep on....and I don't want to write a book in a free republic post...but to jump to the end the evidence all points to a fully perfect all powerful monotheistic personal God similar to the one of Judaism and Christianity. And I am hardly alone concluding this, but am in the company of the founders of modern science.
>>where there is gravity.
Which is, everywhere.
There is nothing and then suddenly there is something before and after the big bang. Maybe the first law of thermo still applies and whatever was before was simply converted to matter after.
And finally, you have to admit that our classical perception of nature is very far removed from the “real world” of quantum mechanics, cosmology. It means that our senses and our perception of nature is wrong. I wonder how far removed it will be in a few decades or centuries.