For me, absolute nothing is a difficult concept to imagine. What is outside of the expanding universe? Is it nothing?
On the very smallest of scales, the vacuum is not at all empty, but teeming with a "foam" of virtual particles: the quantum vacuum.
On the very largest, it's filled with matter. As far as we can tell, the universe is larger than twice the observable size (i.e., ~90G ly), and probably bigger.
Indeed, the reason Big Bang Cosmology is present theory, is because it just so happens to be a configuration of matter and space which exhibits flat curvature and an ever-expanding scale. It's the perfect fit for what we see in our little neighborhood.
That is to say, if you set up a universe with a uniform mass density -- the same everywhere, throughout all of infinite space -- the spacetime will naturally expand along the time direction. Since uniform expansion means more and more velocity at greater and greater distances, there necessarily will be regions that are travelling away from a given point at greater than the speed of light (proper time). Thus, any given point has a horizon, beyond which the rest of the universe cannot be seen; and this fits our observations today.

Tim