The phrase "void space" refers to an empty or unoccupied area or gap. It represents an absence of anything inside that particular space. Full definition
This change in lining material allows for easier transition from content fire to structure fire as the fire has a path into void spaces. (treehugger.com)
Others have densities so low they appear to be almost entirely water ice punctuated with void spaces. (scientificamerican.com)
[2] The flow eventually ceases and drains out of the tube, leaving a conduit - shaped void space which is usually several meters below the surface. (en.wikipedia.org)