What is 'SPACE'

Space.. What is it?

In our atmosphere we are filled with air.
But what about above our atmosphere. Are there any kind of atoms. If no then what is there. This 'Space' is the one that fills our universe and makes its boundaries. And since its boundary i.e. space is expanding, it must something to expand.

So we are sure that space is not empty. Also there is no air, no gas, no atoms. So it is not composed of matter. But since we are sure there is something that makes up empty part of universe, which is not actually empty. So there is something other that 'Matter'. What can be other that 'Matter'?

Also at atomic level, electrons revolve around nucleus at some distance away from it. Then what is there between electrons and nucleus? What is this space composed of?

And you might also think that since our universe's boundary is expanding, in what are these boundaries are expanding?
There might be some different kind of space that separates boundaries. Means there is something else containing our universe which is even larger than it. Then possibly it may have other things, may be more universes. Then what it may be composed of and in what it may also be contained in? This question would go on till infinite but this might not be possible. If this is not the case one possibility is that we might reach same starting point after going too far. May be both are sustaining each other. There can be many possibilities we get answer someday.

Coming back to 'space', one possibility is that space is some for of energy. We are all covered in energy. Our universe and even atoms might be filled up with energy.
Difference of Energy forms can be the separating boundary of our universe and what it is contained in.