Is opposition a concept defined by that thing's mere absence?

Opposition I don't think, really and truly exists. It is merely human beings comparing things to other things and then trying to extrapolate some meaning that is entirely subjective and totally constructed by the human psyche. I also believe that much of what we observe to be opposites is merely the absence of something. The absence of light is dark. In your example, as J said, the absence of 5 things is -5. The absence of "good" is "evil." The absence of the y chromosome is a girl. The absence of that last x chromosome ( where a y stands instead) is a boy. The absence of danger is safety and the absence of heat is cold. As I said though, it's something constructed entirely in the human mind. This is why not everything we observe even has an opposite. Your rock in it's example doesn't have an opposite. It doesn't have any nonobjective meaning to society as a whole, so human beings haven't given it an opposite. To kind of understand what I mean (as I'm sure you realized about me by now sometimes I have trouble putting it down into paper in a simple way) look at animals. They don't wait for the opposite of nighttime to come. They don't notice any light, they sleep. You put a blanket over a bird's cage, and it's going to sleep. It doesn't matter what time of the day or night it is. It's just noticing a lack of light and adjusting itself accordingly, it doesn't compare the two. It sees no opposite. The "opposite" of that of course being bats, who are nocturnal. As you noticed though, a bat only gets thought of as an opposite when you're comparing it to something else. It's "opposition" loses all meaning and significance if we aren't talking about in a comparative way. Opposite is just a comparing mechanism human beings invented to filter the vast amount of information we come across. Most of the opposites come from an observation of absence. If there is no need for an item to have an opposite, because it has no subjective meaning to us, then we don't ign it one.