I think this is a situation where the limitations of language show themselves. IMHO, I would say there's two distinct kinds of "opposite": the lack of the thing (darkness is the absence of light), and the antithesis of the thing (anti-matter annihilates matter).

I think opposites are entirely situational and some things could have more than two distinct opposites. The real answer is "I need more information / in what regard"

Photons are their own antiparticle an therefore the opposite of themselves