Space is what we call 99.999999999% of everything that exists (or doesn't exist) in what we like to call reality.
Space isn't the absense of something, space is the potential for something.
Space has a structure to it that dictates what can be allowed to fill that space.
Space is also a force that attracts stuff. The free space on my harddrives attracts new files all the time, and the free space in my home attracts all kinds of useful stuff as well as junk.
And of course space is the final frontier.