There is evidence that it did occur on earth some two billion years ago, in a few places with high levels of uranium present. As far as the rest of the Universe, we just don't know.
Nuclear reactor
It happened in the center of the universe, for which it is still expanding on the universe.
its really easy for accidents to happen ex. chernobyl USSR in 1986 a fission reaction caused an explosion killing 31 people and giving another 15,000 radiation sickness
Nuclear fission of uranium 238 occurred in the atom bomb dropped on Hiroshima.
It would become an atom of a different element. This can only happen during radioactive decay, nuclear fusion, or nuclear fission.
Strictly speaking nowhere in space, because space has no matter in it. Fusion happens in the stars. Fission may occur in some planets somewhere in the universe but not in our solar system
Nuclear reactor
Lego Universe happened in 2010.
Disney Universe happened in 360.
Entropia Universe happened in 2003.
Fusion only, there are no heavy elements like uranium so there can be no fission taking place
Nuclear fission, the fission or "splitting" of an atomic nucleus, can happen in two basic ways. One is a form of natural radioactive decay called spontaneous fission. The atom just "splits" apart. The other involves the absorption of a neutron, and fission can occur as a result of this as well. Both spontaneous fission and neutron absorption are described in separate questions here on WikiAnswers. You'll find links to them below.
Fission
It happened in the center of the universe, for which it is still expanding on the universe.
Fission and fussion manly occar in a star or a sun. The reason that a star or a sun is bright is because of the heat that is given off of from fission and fussion.
Bust-a-Move Universe happened in 2011.
DC Universe Online happened in 2011.