[Sentis]

what is the universe expanding into?

It is expanding into what Michio Kaku calls Hyperspace. This is basically a higher dimensional space which can only be seen at the quantum level.

[MacDorcha]

well if its expanding into the quantum, why does the quantum plane exist alongside our percieved plane? if it were expaniding, it would be pushing it away, ans we would not see any. when we see the things though, it proves that there is either a leak, or the quantum exists on both sides of the universal barrier. in which case, it would be like pumping a water balloon... underwater. the univers would be expaning into a greater universe.