Atoms, Empty Space and Density

If atoms are mostly empty space, many wonder why matter is not transparent.  Here's a simple answer from sciencefocus.com. 

While atoms are indeed mostly empty space, the void is filled with the electromagnetic fields generated by the electrons within the atom. These affect light waves as they move through materials, preventing them from passing straight through unimpeded. Switching to radiation of shorter wavelength, like X-rays or gamma rays, allows even relatively dense materials to become transparent.