Great question! In short, it's unknown and might be unknowable. Broadly, many think that there isn't anything there. A true absence which is hard for us to comprehend. The big bang wasn't like a bomb exploding in a room but an explosion that created the room through its expansion. Other theories are that we are expanding into the space between universes, or perhaps even pressing against them. 🙂

More space?

if you gaze long into an abyss, the abyss also gazes into you