Since the ancient Greek philosophers conjectured that everything is made of atoms, scientists have been struggling to identify the smallest units of matter. A century ago William Bragg pioneered the use of X-rays to look at atoms. Atomic nuclei containing protons and neutrons, with their orbiting electrons, were discovered. Today, giant accelerator machines, some larger than cities, can probe deep inside the atom. As yet there is no satisfactory understanding of the structure of the fundamental particles, known as quarks and gluons, which are known as hadronic matter and comprise most visible matter in the universe.