I once asked a physicist to explain what the universe expands into, and he gave me the analogy of raisins in bread dough. That the raisins represents galaxies, galactic clusters, etc...and as the universe (bread dough) expands, the galaxies (raisins) move apart. But this analogy assumes that the bread dough expands into the air around it, so I'm still unclear on what the universe is said to expand into. Can anyone clarify? Thanks in advance!