Where does the matter in the Universe come from? We are made of molecules containing atoms of heavy elements made in supernova explosions. Atoms could not exist when the Universe was less than about 300,000 years old. Before then the Universe contained mainly photons and neutrinos, with some electrons, protons and ionized nuclei of light elements such as Helium and Lithium. These were made out of protons and neutrons by nuclear fusion reactions when the Universe was about a minute old. The protons and neutrons were themselves assembled out of quarks when the Universe was a few microseconds old. But where did the quarks come from?
Andrei Sakharov showed in 1967 how these quarks could be manufactured if the elementary particle interactions distinguished between matter and antimatter, as seen in the laboratory, could change the net numbers of quarks, as expected in many theories but never seen in the laboratory, and if thermal equilibrium broke down in the very early Universe. Ever since, particle theorists have been constructing models how this Sakharov mechanism might have worked, when the Universe was anywhere between 0.000000000001 and 0.000000000000000000000000000000000001 seconds old. The Standard Model of particle physics could not have done the job, so many speculative extensions of it have been proposed.
To my mind, one of the most plausible extensions of the Standard Model is supersymmetry (just look at the subjects of my research papers!), so could the minimal supersymmetric extension of the Standard Model have created the matter in the Universe? Sin Kyu Kang and I wrote a paper last week: https://arxiv.org/abs/hep-ph/0505162 proposing that the decays of the supersymmetric partners of neutrinos, ‘sneutrinos’, might have done the job, relatively late in the history of the Universe when it already had the ripe old age of 0.000000000001 seconds. Previously there had been plenty of ideas how the decays of neutrinos or sneutrinos might have produced matter much earlier in the history of the Universe, starting with pioneering work of Fukugita and Yanagida in 1986, but we wanted to do it as late as possible so as to offer a chance of testing the idea in the laboratory. Only time will tell whether this idea could have worked, but it does seems possible that there was a sneutrino our past!