Complex sentence: Most notably spallation is believed to be responsible for the generation of almost all of 3 He and the elements lithium, beryllium, and boron, although some and are thought to have been produced in the Big Bang.
Split into simpler sentences: Most notably spallation is believed to be responsible for the generation of almost all of 3 He and the elements lithium, beryllium, and boron. Some and are thought to have been produced in the Big Bang.