I've more or less understood the universe as the finite (yet infinitely expanding) region of space that encompasses the bubble of matter that the big bang created. There may be an actual border between pure empty space and our universe similar to the skin of a drop of water. This skin could absorb particles/energy (like a photon) where it could somehow convert it somehow to allow this skin to continue growing and/or re-emit it elsewhere within the bubble.

(bear in mind this is just an understanding gathered from the numerous space documentaries I've watched and not from a text book, so I may be way off)