I think this mainly centres on just what should count, here[0] is a paper from 1996 about using astrophysical gamma ray sources to detect the effect of the inte

I think this mainly centres on just what should count, here[0] is a paper from 1996 about using astrophysical gamma ray sources to detect the effect of the intergalactic background light, which is also photon photon interactions. [0]: http://adsabs.harvard.edu/full/1996ApJ...456..124M http://adsabs.harvard.edu/full/1996ApJ...456..124M