When I think of the universe, I imagine a sphere of matter - the physical universe, all the stars, planets, nebulae etc in their respective galaxies - expanding out into an infinite, boundless emptiness. Perhaps this infinite endlessness is filled with other physical universes. It's infinite, so why not?

I realise this model is perhaps not entirely accurate or consistent with theory, but it suits my needs for now. The 'expanding sphere' component is pretty accurate, though.