What is the smallest particle of matter known?
So far it looks like there is nothing smaller inside an atom than quarks. Our experiments look for evidence that quarks have some sort of shape or structure. If they do have a shape or structure rather than just being a point (which by itself is a very difficult concept to understand), then there might be some sub-quark constituents. At present there is little or no evidence of any structure to the quark. Maybe there is though! That might be something for you, the next generation, to look for and try to figure out!
Brian Kross, Chief Detector Engineer (Other answers by Brian Kross)