From the wikipedia page on dark energy, in reference to the total mass-energy of the universe:
The mass–energy of dark matter and ordinary (baryonic) matter contribute 26.8% and 4.9%, respectively, and other components such as neutrinos and photons contribute a very small amount
Why do photons contribute such a small amount?
The baryonic matter in the universe is the remains of what is generally quoted as a billion to billion-and-one ratio of antimatter-matter. If the billion on one side collided with the billion on the other, and all that mass-energy was converted into photon energy, why isn't there significantly more photon energy in the universe?
I understand that photons dominated the mass-energy of the universe in the photon epoch shortly after annihilation... but since the CMB still stretches to every point in space, I don't see how that energy became 'lost', and photons now only make up a small amount of the total mass-energy.