How to search for the space around the particles

Hello everyone, I want to search for the voids around the specified particles to add small particles during the DEM simulation. Is there a way to search for the space around the particles?

You can divide the space around that specific particle into small blocks and check which block is void.
The particle’s neighbor list may help your calculation.

Thank you for your answer to my question,
I can understand what you mean,
But in which file can the neighbor list be found?
Thanks again