Just thinking of Hubble's simple expansion of the universe or galactic recession (ignoring inflation). How is it possible or practical to say that the universe is expanding? For something to expand, it has to expand relative to something else (if not, then inertial frames would not be relative). But if everything was expanding, then nothing would be. You could always say either the whole universe was expanding, or shrinking or neither expanding or shrinking, and the effect would be the same. One teacher told me that it is just the space between the galaxies that is expanding, but that made little sense, since our galaxy would soon be crushed by the space that surrounds us.