Let us assume again, that a person has a collection of 100 books. The books are in a shelf in way that between some books there is an empty space. Probably we can say, that the empty space is part of the collection at least visually considering the whole, although that empty space is something different than a book.
In general an empty space can be filled with a book, so that the book is surrounded by an empty space. If no empty space existed, a book couldn’t be taken away from the collection of the books (or in this example put a book to the shelf).
Therefore empty space is part of every collection of books..