Particles have all kinds of theoretical qualities, capabilities and even identities. Most of them have been theorised before they were proven to exist. The neutrino was one such particle, and its journey from the core of the Sun, or from the death of a neutron star, or even from a nuclear power plant, is the object of great scrutiny all around the world - from the giant cavern of SuperKamiokande to the underwater glass array of ORCA, neutrino-dedicated experiments spatter the planet with devices of ingenious complexity. In unfathomable quantities easily measuring billions, neutrinos stream through your body - and everything else - all the time.
Despite the impressive amount of supreme technology, little is known about the neutrino. Originally they were thought to have no mass at all, since they pass through matter, for the most part, unhindered and unseen. The beauty of these experiments lies in their dedication to neutrino detection, and in recent years the theory of No Mass was debunked, clouted soundly round the ear by the discovery of oscillation in 1998. For neutrinos change as they fly from one form into another, and in order to do this, they had to have mass, and that mass naturally had not only to be accounted for, it had to be measured. So it was.
While they are changing from one orientation to the next, neutrinos are said to 'disappear'.
Only, in the discovery of their mass, another conundrum surfaced. CP violation is a headache for physicists, it's an unknown quantity that mucks about with equations and creates more problems than Charge and Parity (the things that are violated) can solve. So in order to potentially quell the dichotomy of CP violation, a man called Ettore Majorana proposed the unthinkable - that the neutrino was, quite possibly, matter and antimatter at the same time. Like the oscillating neutrino, Ettore disappeared under mysterious circumstances.
Without going into dark matters too deeply, we're clearly on the cusp of something big. And the GUT is telling us, if it's ever to be believed, that five forces plus a Majorana particle could open the door to new physics that wipes the Standard Model clean off the slate in terms of progressive critique.