Sometimes they're mass. Formulas of quantum mechanics were devised to cover all qualities. The mysterious bits remain unknowable. The unreality of matter can be seen in neutron stars and black holes. When remains of a supernova collapse under colossal gravity, electrons of each atom are compressed into the nucleus, where they merge with protons to form a solid mass of neutrons.
Nothing is lost except the void within the atom.
The compacted neutrons weigh 10 million tons per cubic centimeter. Too little to see with a microscope. We are almost entirely vacuum. Even packed neutrons aren't solid. If a supernova's remains are three times greater than our sun, gravity's pull is too strong to stop at the neutron star level.