It's not just opinion. See the two photon physics article on wikipedia? It concerns actual gamma-gamma pair production, and is fairly representative of what you've been taught. Now pay attention to this excerpt: "From quantum electrodynamics it can be found that photons cannot couple directly to each other, since they carry no charge, but they can interact through higher-order processes. A photon can, within the bounds of the uncertainty principle, fluctuate into a charged fermion-antifermion pair, to either of which the other photon can couple." This says pair production occurs because pair production occurs. It's a non-answer tautology, and I'm afraid it's wrong. Gamma-gamma pair production occurs because photons do interact. We have direct evidence of gamma-gamma pair production. What we have no evidence of, is single photons spontaneously morphing into an electron-positron pair which then miraculously annihilates back into a single photon. What you've been taught misinterprets QED, disregards what Feynman said, and treats a calculation method as reality when it isn't. Do you seriously think an electromagnetic field consists of short-lived photons flitting back and forth? Do you seriously think there are tiny flashes of light within a hydrogen atom, or that a magnet shines? Virtual photons are field quanta, or "chunks of field". Like you've divided a field up into little cubes. They aren't actual particles flittering around, and they aren't the same thing as the vacuum fluctuations of the very weak Casimir effect. And I reiterate, we have no evidence of single photons spontaneously morphing into an electron-positron pair which then miraculously annihilates back into a single photon.