The results of this onslaught on the realm of 'inner space' have been spectacular. We know that matter has a deeper layer, revealed only as we probe in studies of cosmic rays. The proton, neutron, pion, kaon, lambda and many other subatomic particles are themselves complex structures, based on only a few, more basic particles - the quarks and their corresponding anti-quarks.
Six types of quark are known and have been named: up, down; charm, strange; top and bottom. These quarks combine in groups of three to form the proton, neutron, lambda and related particles called baryons. The quarks can also bind with antiquarks to make particles such as pions and kaons, which are collectively known as mesons.
The electron and muon, on the other hand, are not made from quarks but appear,as far as we can tell, to be indivisible. They belong to a separate family of particles called leptons, which also include a third still heavier charged particle, the tau, as well as neutrinos - particles that are almost massless, neutral and difficult to detect.
Just as important as the quarks and leptons - the building blocks of matter are the forces that act between the particles and mould them into the forms of matter we observe. There appear to be four basic forces at work in matter - gravity, the electromagnetic force, the weak force and the strong force.
Gravity is the weakest of the four, but acts over great distances, binding stars and galaxies together. The electromagnetic force is stronger and is responsible for holding atoms and molecules together. As with gravity, its range is infinite. The weak force and strong force are by contrast limited in range, and operate only within the dimensions typical of an atomic nucleus. The weak force causes certain forms of radioactivity and underlies the nuclear reactions that fuel the Sun. Last but not least, the strong force - the strongest we know of - binds quarks and antiquarks together within the particles we observe. The strong force seems to act in such a way that quarks are always locked inside these more complex particles, so that we have never observed a single free quark.