"The leading theories of matter do not favor the idea that subatomic particles are like little billiard balls of differing masses. For example, string theorists speculate that all particles are made of the same stuff; namely, vibrating bits of string. Except that they each vibrate at different frequencies. Problem is, string theory is purely theoretical and really falls more in the mathematical domain than the scientific domain, inasmuch as there is no supporting evidence for the theory. If it does turn out to be true, even the neutron is mostly empty space because the string is supposedly one-dimensional, with a theoretical cross section of a Planck length. "