What is the name of the philosophical idea that, if we went back to watch the big bang unfold, we would see the universe get smaller until we reach something infinitely hot and dense

This is a completely new concept to me, and I thought it was so intriguing. If the universe started from a single point at a single time, that means it was infinite at a single point in space, and as it expanded it started becoming finite. This leads me to wonder what happened to all of the matter left over from the space that was reasoned away.