There should be space OUTSIDE our universe - according to basic reason and the options available.

Locksley wrote: I fricken love how tiny we are. The pale blue dot...

"What counts is not necessarily the size of the dog in the fight, but the size of the fight in the dog." -Dwight D Eisenhower

I love our little blue rock too

Here's my theory: the universe is NOT expanding. The universe is just...there. It's just a black space filled with galaxies and particles and atoms and molecules and dark matter. There are no walls to it...there is no ending...it's just all the space that contains these different things. The galaxies are moving farther apart and farther away possibly from some central point, but I don't believe the universe itself is expanding. I don't believe it can because of my belief of what it is. I don't believe it has a wall that separates it from something else. If it did, wouldn't that mean that it's not really infinite after all?

The universe is defined as energy (that is any sort of EM-waves, including light, matter, and the few other things we don't understand yet) and space (which is mere emptiness). At the point where there is no longer energy in space and empty space begins is the "wall" of the universe. The "size" of the universe is obviously the amount of space that has any energy within it. Since light uses to travel farther away from its origin point (and occasionally adjust its direction slightly), one can almost expect the size of the universe to grow constantly. There are other phaenomena the most plausible explanation for which is not even the constant but the exponential increase of distance between the "walls".
So yes, for all we know there is space outside the universe, the universe is not just there but has a more or less accurately known size which is changing.
Now, if that is true or not is matter to discussion. It is held to be true, because the empirical evidence we have points to it and we hold that again to be true, just because if we'd question any idea the world confronts us with, we might as well stop researching, thinking or doing anything really and perhaps evolve into some sort of plant, that (as far as we can tell) wouldn't care at all about the nature of our world.

The way I see it, the amount of energy available is a constant... So the universe never really expands... It merely gives the illusion it's expanding because the space pane keeps on expanding (at the cost of time perhaps?). It eventually reaches a point where there is no more energy available, (or runs out of time?), everything starts collapsing on itself (big rip), "pressure" eventually builds up and we get a new big bang.

Anyway, I don't find it hard to believe the universe does not expand "into something", but "out of itself".

The problem is that, you consider the universe as something flat. There is no wall in our universe.

Let me ask you a question, imagine You are going to direct in to space, after a while (and that while is VERY, VERY HUGE!) you'll be back on the first place. Just the way that you go on earth, you'll be back to the first place as well.