These are clues to a riddle:

Clue 1: Without nuclear reactions, it would not exist on Earth.

Clue 2: It can be seen but not touched.

I will see if there are more clues later. Thanks.:)

YAY! For answers, but it says it wouldn't exist on Earth, so it must be on Earth now. (?)

The sun rise (or sun set) - you can't touch it, and the light from the sun depends on nuclear reactions.

Nuclear reactions release energy as light, heat, sound and radioactive energies right? Then I suppose that light, which can be seen, contrary to others is the solution.

Its pretty the same thing as ebaines said but perhaps the riddle wouldn't accept the sun rise/set because it is not on the Earth but can be SEEN from Earth.

Hope that's the solution.

Here's another guess - a rainbow. Again, the light comes from the sun, and you certainly can't touch a rainbow.

Sunlight!