4. Dark Energy.
This is the mysterious force causing the expansion of space. No one has ever seen this force.
Well, if your question is how light affects plants,
then you would want to design an experiment that plays aruond with the amount of light a plant gets
thus the thing changing (or variable) would be amount of light
Currently, many researchers are still working to find them.
Currently , its only theorized and they should be based on the quantum and particle theory
hope this helps
