What does the Universe expand into?

I was reading reddit and came across this comment to the question above.

the growth is uniform?

In general. There are local areas where galaxies are gravitationally bound, but the expansion of spacetime means other groups become increasingly more distant.

http://en.wikipedia.org/wiki/Metric_expansion_of_space

the growth can be measured?

This is the most solid area of the cosmological science. We've known about the expansion since the 20s, and despite its unintuitiveness, discovered that the rate of expansion is increasing.

there is no central point from which the expansion is happening?

There are no privileged frames of reference. Within our 3-dimensional experience, there is no center. Every point is expanding away from the others equally.

But 4-dimensionally, you can view the instant of the big bang as the point in spacetime from which everything expands. As we move along the axis of time, the spacial dimensions all increase.

Alternately, if you view our universe as the surface of a hypersphere, the center of that sphere can be viewed as the central point; however it is "outside" the space we occupy.

The Bad Astronomer attempts an explanation: http://www.youtube.com/watch?v=nw5W3CszeAI

that still indicates that the balloon is expanding IN something... know what I mean?

Not really; that's where the analogy breaks down. Space and time are both metrics internal to the Universe. They only make sense in reference to things within space and time. You can have no "space outside space" or "time before time".

The Big Bang wasn't an explosion of matter into an already-existing expanse of space. Space and time themselves emerge from that singular point, along with all the matter and energy in the universe.

If the universe exists at its origin point as a singularity, then that is how it exists relative to anything "outside". Everything else happens "inside" and has no bearing on what occurs "outside". One model is that our universe is one of an infinite number of "bubble universes" that simply pop into existence as a consequence of quantum vacuum fluctuations.

http://en.wikipedia.org/wiki/Chaotic_inflation_theory
http://www.lifesci.sussex.ac.uk/home/John_Gribbin/cosmo.htm

Here's a few links that may address your question better than I can:

http://curious.astro.cornell.edu/question.php?number=274
http://curious.astro.cornell.edu/question.php?number=651
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/011021a.html
http://www.sciam.com/article.cfm?id=according-to-the-big-bang-1999-10-21&catID=3

http://reddit.com/info/6iz44/comments/c03z5jd