“the fact still remains that the phenomena of darkness exists, or there would be no need to define it.”
The boy was not arguing God’s existence, but God’s nature. The professor said that the existence of evil showed that God is evil. The boy rebutted that logic by giving examples of two things that are not positive forces in their own right, but are rather only the absence of other forces. He then said that the existence of evil is just such a thing.
Rather than demonstrating that God is evil, therefore, the existence of evil is merely the manifestation of God’s absence.
(This is a proposition to which I do not subscribe, by the way. The boy’s argument does not suffice to rebut the professor’s assertion; that must be done on other grounds.)
My point was that just because something can be defined as merely the absence of something else, doesn’t make the thing you are defining any less real. Since, as the quote from Isaiah that posted demonstrates, God has taken credit for not only creating light, but also darkness, even if it is only the absence of light. He created the phenomena of the presence as well as the phenomena of the absence, and both are equally “real” phenomena, no matter how you define them.
You are right that the boy’s argument doesn’t really rebut the professor’s assertion. Even if evil is only the absence of God from our hearts, an omniscient God must have known that condition would arise if he created beings with the freedom to choose to turn away from him. So even if the boy can successfully argue that God didn’t created the absence, the professor could reply that God created the opportunity for the absence, knowing full well that the absence would necessarily occur.