When looking at the energy count in the Universe, on one hand we are told dark energy 68%, dark matter 27%, regular matter 5%, neutrinos and photons a very small fraction (trace like). https://en.wikipedia.org/wiki/Dark_energy

On the other hand we are told, when regarding matter- antimatter imbalance that in the early Universe there was ~ 10 billion antimatter particles for 10 billion +1 matter particles. The 10 billion annihilated together and the matter particle remained.

Since this annihilation produces photons, should there not be about 10 billion (x2) times more energy in the form of photons than in the form of regular matter?

The loss of energy by photons due to expansion does not account for the difference between these two results, does it?

What is the explanation for the apparent discrepancy?

  • $\begingroup$ The only reason the Big Bang (BB) model needs anything "dark" is because the model only uses 1/3 of the baryonic mass of the Universe. If it generated all the baryonic mass during the during the BB, then Cosmic Microwave Background (CMB) would be 15 K. Instead of using "hot" baryonic mass in the BB, they use He - despite the fact He can be produced in stars. $\endgroup$ Apr 17, 2019 at 0:23
  • $\begingroup$ But still, as for the question, shouldn't the energy of the "two billions" annihilated particles be much more than that of the excess particles? It is long time that I wanted to ask the basically same question, except for that I am aware of the different scaling of mass energy density and eras as in the answer by @Ben Crowell. $\endgroup$
    – Alchimista
    Apr 17, 2019 at 8:18

1 Answer 1


The loss of energy by photons due to expansion does not account for the difference between these two results, does it?

Yes, it does. The universe had a light-dominated era, then a matter-dominated era, and is now in an era dominated by dark energy. The reason for these changes is that each form of matter has an energy density $\rho$ that scales differently with the scale factor $a$. For light, $\rho\propto a^{-4}$, because there is both a dilution of the number density and a redshift. For nonrelativistic matter ("dust"), $\rho\propto a^{-3}$, and for dark energy, $\rho$ is constant.

  • $\begingroup$ thanks so it is really the streching of space increasing the wavelength of photons and thus reducing their energy that accounts for this. Three further questions. 1) do you need inflation, for the numbers to add up? 2) where does the lost energy of the photons go? 3) is energy conserved as universe expands? $\endgroup$
    – mim zim
    Apr 18, 2019 at 10:15

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