The way I am thinking about this is thinking of a bomb floating in the middle of a room until it is set off. Once it explodes, the room doesn't expand. Instead the contents of the bomb does.

Is that what universal expansion is? The matter, particles, energy, etc simply moving away from the starting point in all directions. Or can the universe be thought of as a bubble that contains all those things, but is itself expanding and taking along its contents with it?