The area contained within those intermolecular areas are full of...nothing nope it's not air it's closer to being defined as a vacuum but it isn't vacuum because it is filled air would not for into these spaces because "air" as we know it is many different element in their gaseous form the space is there because all atom vibrate and also carry certain atomic charges. These charges are quite often not NET zero but there are still charges and partial charges present in the molecules. So think of that area as each molecule's "personal space".