Space is, in its simplest terms, the geometry of three dimensions. In mathematical theory, spaces with more or fewer than three dimensions can be described, although these cannot exist in the real universe.
Outer space is the physical realm beyond the Earth's atmosphere.
"Space ... the final frontier" is the opening line of the old Star Trek show.
On the ontology of space
What is space made of? This may seem to be an odd question, since space seems simply to be space: it isn't made of anything but itself. But, if space is not made of anything but itself, then it seems we directly-and-simply (i.e., immediately) are perceiving something which exists purely in terms of itself. But, is space self-existent? If space is self-existent, then, in light of the foregoing, it seems we have immediate perception of something (in this case, of space) that is self-existent. In short, it seems we have a priori knowledge of something that is self-existent.
But, we seem immediately to perceive not only of space as such, but that space is omnipresent. Further, we seem immediately to perceive a contrast between space and matter, in that matter seems to be a subject within, and of, space, whereas space seems to transcend matter. For example, while matter seems to 'have' space, yet, when matter is moved out of one location and into another, we do not perceive that space has been acted upon as a material substance like water, but, rather, as an immaterial substance like God. In other words, it doesn't seem that any 'piece of space' is ever displaced when a material object is moved into it, even though that material object seems already to have its own movable 'space'.
So, space seems both omnipresent and transcendent. But, space also seems indifferent and impotent. So, if space is actually something unto itself, having no dependence on anything else, then how can it actually be that by which all else obtains? It seems that space is incapable of being the ground of any synthetic material entity, including motion. Because, matter seems not be reducible to space: matter does not seem to be made of space.