About dualism, ever heard of heat? Heat is warm, and the opposite of that is cold... but what is cold? Well, the absence of heat. But when do we have a absence of heat? Theoretically at -273.15 degrees celsius, but we've never been able to reach that and we don't think it exists naturally anywhere in the universe. Yet we call a lot of other things, that are not absent of heat, cold. Cold is not actually an absence of heat, it is a lesser degree of heat. The whole universe is heat, because there is no absence of heat. We can measure the exact amount of heat without relying on the concept of absence of heat.
However, "the absence of x" is a really cheap way to describe something that is the opposite of x. Being absent, or not existing, is the opposite of existing and nothing else. Let's take matter for example. Matter is this thing with mass that takes up space. Then what is the opposite of matter? Well, the absence of matter. But then we have antimatter, which is supposedly the opposite of matter. The absence of matter is not the opposite of matter in this case, it's the opposite of the presence of matter. To have matter is the opposite of not having matter, but matter itself is not the opposite of not-matter. We can apply this to light as well: The presence of light is the opposite of the absence of light, but light itself is not a opposite of not-light.
There doesn't have to be a opposite for something to exists, and an absence of something does not qualify as an opposite to it either.