So, are we assuming the universe is infinite? Otherwise there'd be a border to that.
What isn't infinite is the observable universe. But the observable universe is just the view we have, its boundaries are defined by the observer. In fact my observable universe is different from your observable universe, by an insignificant amount.
When physicists talk about the universe, it usually means the observable universe. Simply because for us, that's all we have.
Wikipedia for one says the following. There is no mention of infinity.
"The Universe is all of space and time[a] and their contents,[10] including planets, stars, galaxies, and all other forms of matter and energy. While the spatial size of the entire Universe is still unknown,[3] it is possible to measure the observable universe."
A finite universe would be similar, just adding a dimension.
We have no clue what is beyond the explosion itself and, for intents and purposes, that expanse is infinite by current understanding. Does that expanse contain other explosions? Don't know, there hasn't really been an observed overlap so far.
You could imagine that the explosion is happening on an extra dimension and our three dimensional universe is the wave front of that explosion. Except that the math doesn't actually require the extra dimension to exist nor there is any evidence that it does.
edit: also the cited diameter is only for the visible universe. The universe is currently understood to be infinite.
[1] my understanding this applies only to points of the universe that are not otherwise gravitationally bound with each other.
I don't know the details, but I was told that with these assumptions, proving that the Universe is infinite is only a matter of mathematics.