Screw finding out what the universe is expanding into. What makes up the space between two points?
You’ve got a string here and a string there. What exactly is separating them?
That’s the nothing I want to know about.
I disagree that it doesn’t exist at all. “North of the north pole” is nonexistent because it’s a contradiction in terms, because of how “north pole” is defined. However, what is outside our universe is not nonexistent by definition, however, it is unknowable by definition. This makes it the territory of philosophy, not science. There are theories of other universes outside ours, but AFAIK they are not testable.
I do not know what contemporary thinking is in the scientific community about whether we are “Boxland” in some higher dimensional “Hyperland” universe that we can’t detect, but in my reading of popular work on the subject I’ve never seen it suggested. I have read that string theory requires a bunch of additional dimensions but they are rolled up so small we don’t notice them, much different than the Flatland analogy.
Fair enough, but that’s just different from the ‘nothing’ I’m going for, which is the state of the absence of everything, including the existence of anything. Not a blank slate - but instead: no slate at all.
If existence is a property, then nothing can’t exist – for to have properties is to be something (that thing which has those properties), and hence, not nothing.
In a sense, it’s always non-sense to talk about nothing – whenever you talk, you talk about something, because if you don’t, your talk is meaningless; but then, when you talk about nothing, you actually don’t!
If you define the Universe as “all that exists”, then “outside the Universe” is just as much a contradiction in terms.
What about talking about talking about nothing? 
Not when the context is (as provided by the OP) an expanding universe (ie the observable one). But I’m just being pokey.
that is a dilly of a melon scratcher.
Relax. What is mind? No matter. What is matter? Never mind!
No, Chronos is correct. Even with an expanding universe “outside the universe” is still a meaningless phrase.
Nope, ‘universe’ could mean ‘our pocket of the anthropic landscape’, among many other things.
ETA: In fact, ‘expanding into’ strongly implies the context being the observable universe, which is very different from the definition suggested by chronos.
Well, yes, “if.” Are you saying it’s not possible that there are other universes?
Thanks all, for your responses. 
The fact that the observable universe is expanding is completely unrelated to the fact that the Universe as a whole is expanding. And from the OP’s phrasing, I think it’s clear that he means the Universe as a whole, since what’s beyond the observable universe is not, in any sense, “nothing”: There’s plenty more universe out there.
Huh? We only know that the observable universe is expanding (by definition). We don’t know that the ‘Universe as a whole’ (whatever that means) is expanding. There are many things the OP may be interested in besides being told his question about what is outside the universe is purely definitional, for example branes moving through the bulk.
True. We can only extrapolate from what we observe so far. We can see back about 13.7 billion years but the Universe is much larger than that. The very earliest matter from the Big Bang has already accelerated out of sight and we can never - never - see that.
For me the balloon analogy works. The Universe is like the surface of an expanding balloon. Nothing inside. And nothing outside.
Why would there necessarily be a difference between the empty vacuum of space and the nothingness into which the universe is expanding? Are we assuming that space is really some form of “dark matter” rather than an absense of matter?
Nothing is easy to define. It is a compound word.
No Thing.
It is a definition of what it is not, and it is not a ‘thing’.
Where there is nothing, there are no things.
Therefore when the Universe expands into where there are no things, there are now things there.
Next week’s lecture, why are “before” and “after” based on nautical terms? (Be Fore, Aft Er)
This is beyond my little knowledge, some I’m guessing here.
The difference is that the very three dimensions are part of the universe. Empty space has three dimensions - x, y, z (well, more). Nothingness does not possess dimensions. You can’t travel into it. It doesn’t exist. It’s “nothing” - not even dimensions.
Candyman74:
That’s just semantics. Since the universe does expand, it clearly is being “travelled into”. I’ll grant that the fact that matter and energy and light has not yet reached that which is not within the universe means that distance within cannot be measured, but if it can be expanded into, it exists as empty space.
But the expansion of the universe is not stuff ‘travelling into’ something (or nothing, as the case may be). Space itself expands; it’s not the case that there already is space out there, empty or otherwise, that is being expanded into. Rather, the space of the universe is all the space there is, there’s nothing ‘beyond’ (or rather, there’s no beyond).
Empty space is indeed different from nothing in that it has properties, like being three dimensional, or of Lorentzian signature, or curved (or flat, as the case may be). Nothing has none of these properties.