The Greeks believed the fundamental constituent of matter was the atom. This stuck for more than 2000 years until those even smaller elements, protons and electrons, were discovered.
In 1964 the quark revealed itself. In the 1980s, string theory, a mathematical representation where particles are considered to be strings of vibrating energy, led to five different, seemingly unconnected theories. In the 1990s, M Theory unified them.
But what is the M? Magic, mystery or matrix? Membrane, mother or master, even missing or murky?