Photon–photon interactions limit the spectrum of observed gamma-ray photons at moderate cosmological distances to a photon energy below around 20 GeV, that is, to a wavelength of greater than approximately 6.2 × 10 m. This limit reaches up to around 20 TeV at merely intergalactic distances. An analogy would be light traveling through a fog: at near distances a light source is more clearly visible than at long distances due to the scattering of light by fog particles. Similarly, the further a gamma-ray travels through the universe, the more likely it is to be scattered by an interaction with a low energy photon from the extragalactic background light.