The Big Bang is not a physical explosion and the universe is not a balloon. All space was condensed together, but space is (to our current ability to measure it

The Big Bang is not a physical explosion and the universe is not a balloon. All space was condensed together, but space is (to our current ability to measure it) topologically flat and infinite. It was still infinitely big in every dimension, it was just unfathomably dense. The expansion that's occurring is that the space between things is growing. But it's every space between everything all at once, everywhere. There is no physical equivalent.