A star like our Sun burns Hydrogen to Helium. More massive stars burn Helium to make heavier elements. The heavier elements were produced in Supernova explosions. When a massive star can no longer support itself through burning Silicon to Iron, its core collapses. Some of this energy goes into producing fast neutrons which bombard material in the outer layers of the supernova.
Neutrinos are mass-less subatomic particles that couple weakly to ordinary matter.
Neutrinos are released through radioactive decay of matter and are also produced through collisions of matter and anti-matter in the early universe. When matter and anti-matter collide, they annihilate producing radiation. It has the same mass as an electron, but carries a positive charge. Positrons are also produced at accelerators and through radioactive decay. Most cosmologists believe that before the big bang the universe underwent a phase of rapid expansion called inflation .