I think this is related with the notion of entropy. If something is constant, necessary and omnipresent, it seems to be undefinable as there is nothing to compare it with. A constant, with no entropy, means no information and no knowledge, nothing to say about it.
However, we don't need the "opposite". What are the opposites between vacuum, water and air? What is the opposite of red? I think the best example is gravity.
Magnetism and electrical charges have two polarities, so the force can be attractive or repulsive. In quantum mechanics we can see a lot of symmetry, but gravity is puzzling because it seems not to be symmetric. Both matter and antimatter have the same gravity if I'm not mistaken.
However we have fairly good definitions for gravity. We have now gone to the space and we have a notion of "weightlessness", but:
- there is gravity in space as well, although it's small and other forces like inertia may be more significant.
- gravity was well defined before, we didn't need to go to the space to define it.
How could we study gravity before? Because it's not constant. For instance it is weaker in the mountains. Therefore we don't really need the opposite, but variability is definitively interesting and important. With a differential in gravity we can study it's causes, define gravity as dependent on those causes, etc.
If gravity was constant, like the speed of light, we could still know it and have some information about it, but the greater the entropy (contingency, variability, etc.) the more we can know about something.