In solids atom or molecules are arranged in manner that they have no gap between them i.e they are very tightly packed and that is why they are celled rigid.
But electrons have nothing to do with the packing of the molecules, Free electrons are those which are loosely bounded with the nucleus, so they need very less energy to eject from the atom and to conduct electricity.
Free electrons do not travel through the voids they travel across the bulk and surface of the conductor.