If you mean all chunks hidden by fog-of-war then here is a console command. Save first, single player only, use carefully, not responsible for loss, etc...
/c local p=game.player local s,n=p.surface,0 for c in s.get_chunks()do if not p.force.is_chunk_visible(s,c)then s.delete_chunk({c.x,c.y}) n=n+1 end end game.print("Deleted: "..n)
