A couple of questions that have always bothered me...
What is the Universe expanding in to?
And
What exactly IS the universe?
I've got this far...
If the universe is infinitely big, then the answer is simply that it isn't expanding into anything; instead, what is happening is that every region of the universe, every distance between every pair of galaxies, is being "stretched", but the overall size of the universe was infinitely big to begin with and continues to remain infinitely big as time goes on, so the universe's size doesn't change, and therefore it doesn't expand into anything. If, on the other hand, the universe has a finite size, then it may be legitimate to claim that there is something "outside of the universe" that the universe is expanding into. However, because we are, by definition, stuck within the space that makes up our universe and have no way to observe anything outside of it, this ceases to be a question that can be answered scientifically. So the answer in that case is that we really don't know what, if anything, the universe is expanding into.
The universe is commonly defined as the totality of everything that exists including all physical matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. More customarily, the universe is defined as everything that exists, has existed, and will exist. According to this definition and our present understanding, the universe consists of three elements: space and time, collectively known as space-time or the vacuum; matter and various forms of energy and momentum occupying space-time; and the physical laws that govern the first two. A related definition of the term universe is everything that exists at a single moment of cosmological time, such as the present, or 'now' as in the sentence "The universe is now bathed uniformly in microwave radiation".
When you attempt to comprehend all of this, it makes you feel quite small.
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