The entire universe can be represented by the few particles included in this table.* It would be even more amazing, then, if these 19 were made of the same elemental constituent. If superstring theory turns out to be true, this will be the case. According to the theory, every one of the "fundamental" particles are made of identical strings, with the only difference between the strings being their vibrational patterns (see Resonance in Strings for more on this).
The Standard Model, Plus Gravity
Superstring theorists believe they are on their way to developing a framework that incorporates both general relativity and quantum mechanics. If they succeed, they will have discovered a theory that solves the greatest problem in physics—that of unifying the laws of nature.
*The table lists 19 known particles (17 of which have been experimentally verified, though some of these indirectly). The matter and forces we experience, however, can be represented by about half that number. This table does not list any antiparticles.