Nothing: a vacuum is the lack of matter.
In air, light reflects off gas molecules and hence, "fills more space".
other organs of the body get into that space
There is nothing that fills the space between neutrons and electrons. There may be a couple of other particles, but they do not come close to filling the space.
Essentially yes - it is empty space. there may be some stray particles now and then but for the most part it is empty.
Space is a vacuum. Sound cannot travel in vacuum
Air.
Volume
Not all occupies space, because air does not take space.
A hard vacuum or interstellar space.
well space is like a vacuum you cant escape it and sound wave do not travel in space but space isn't literally a vacuum so they move by rockets and the gravity of earth
Gas diffuses in space.
That is called the volume.