Transcript: It’s easy to forget, since the night sky is so dark, that the universe is filled with radiation. The cosmic microwave background represents photons from the big bang stretched by the expansion of space with their energy reduced to microwaves. There are vastly more photons in the microwave background radiation than there are particles in the universe, a hundred million photons for every hydrogen atom, so the total in the universe is ten to the power eighty-eight photons. What is the equivalent mass of this huge amount of energy? Using Einstein’s E = mc2 it’s possible to calculate it, and the result is an exceedingly low amount of equivalent mass. The energy of these photons is very low. The radiation density of the universe corresponds to twenty thousand times less than the critical density, far lower than the mass density of dark matter or even normal baryonic material. Even though the universe is flooded with photons, matter governs the behavior of this universe.