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?
There is a difference though. With a bomb, there would be a shock wave. In the universe, we don't observe such a shock wave. With a bomb, the velocities of objects would diminish as one goes further away from the point where the bomb exploded due to the decrease in energy further away from the explosion. In the universe by contrast, we see that the velocities increase with distance. As a result of that, one cannot identify a point where such an explosion would have occurred. One would observe the same increase from every point in the universe. Therefore, it is concluded that the universe itself is expanding.
What it is increasing is the distances between the galaxies measured in the comoving rest space (the 3-space orthogonal to the world lines of the galaxies). This phenomenon at large scale is homogeneous and isotropic in this sense it is an universal expansion at large scale. It does not make sense instead to think of an universal expansion of everything at every scale: local rulers cannot expand otherwise we could not measure the expansion.
