if the universe is expanding, what is it expanding into?

Question: if the universe is expanding, what is it expanding into?
- Keywords:
  expand,
  expanding,
  expansion,
  space,
  universe
Asked by a09jthwaites to Indi, Jarv, John, Ken, Vicky on 15 Mar 2012.
- John Prytherch answered on 15 Mar 2012:
  
  It isn’t expanding INTO anything, it’s just expanding. To understand this, imagine that the universe is the surface of a balloon. When you inflate the balloon, the surface of the balloon expands. This is kind of what is happening in the universe, except in three dimensions.
  
  This is a tricky thing to understand (for me at least!), but astronomy and cosmology are amazing things to study.

Question: if the universe is expanding, what is it expanding into?