Quantum Mechanics is the study of the very small. It began in the 1920's and contiues to be the best model of small-scale behavior. One of the fundamental postulates of quantum mechanics is that all particles behave in some regard as waves (this discovery was stimulated by Einstein's work on the photoelectric effect). This leads to the discovery of the Heisenberg Uncertainty Principle, that one can never know both the exact position and momentum of any particle. In fact, particles can only be expressed as probabilities! This view of the universe is a bit unsettling to many, as it seems to suggest that "reality" as we know it is, if you look closely enough, nothing.