Many anatomic spaces are potential spaces, which means that they are potential rather than realized (with their realization being dynamic according to physiologic or pathophysiologic events). In other words, they are like an empty plastic bag that has not been opened (two walls collapsed against each other; no interior volume until opened) or a balloon that has not been inflated.
Many anatomical spaces are potential spaces, meaning they hold the possibility of being filled but not yet actualized (their realization is dynamic based on physiological or pathophysiological events).  This concept can be compared to the state of an uninflated balloon or a closed plastic bag with walls pressed together.