I learned a lot about this question from
this thread, which is already back on Q&A page 5. It's a rather long thread. If you don't have much time, you might start on page 4 (of 5), where
Spaceman Spiff points out...
Tim Thompson, above, already linked to this article, but I think it's worth pointing to again: "Expanding Space: The Root of All Evil?" (download the full pdf file, either from that link or directly, here). Try it - you'll like it! (or rather at least learn something new)
I loved this line from the paper....
The balloon-with-dots or bread-with-raisins analogies, like any analogies, are useful so long as we are aware of what they successfully illustrate and what constitutes pushing the analogy too far.... They correctly demonstrate the effects of the expansion of the universe, but not the mechanism. That they fail at some level is hardly surprising: we’re representing 4-dimensional pseudo-Riemannian manifolds with party supplies.