Could all the energy in all the photons in the universe account for dark matter? I was hit with fridge logic, and I am curious:
Is it possible that the gravitational influence of photons inside of galaxies (And all throughout the universe) could account for dark matter?
Photons would be most concentrated close to the core and inner galaxy... And have a lessening concentration as they expanded away from galaxies due to the inverse square law..
I am struggling to understand the math involved, because I've not yet reached that level in my education.... But I'm really curious how the total photons stack up vs the measured dark matter in the universe?
 A: The problem here is the contrasting effects the two forms of matter have. 
Photons are massless; they create negligible gravitational fields. 
Dark matter, on the other hand, is about 85-90% of all mass in the universe. It's responsible for holding galaxies at constant rotation rates past certain radii (see Galactic Rotational Velocity Curves). 
Photons, on the other hand, provide a repulsive force that we call radiation pressure. 
In terms of contribution to the energy density of the universe:
Let the critical density $\Omega_c=1$. This is the energy density of flat space, and provides an accurate description of the large-scale universe by current measurements. 
$\Omega_m$ is the energy density contribution of barionic matter. It's approximately equal to $0.3$. Of that $0.3$, visible matter contributes about $0.05$. 
$\Omega_\Lambda$ is the energy density contribution attributed to the cosmological constant $\Lambda$, and is about $0.7$, or about 70% of the energy in the universe. 
You might have noticed that $\Omega_m + \Omega_\Lambda \approx 1 = \Omega_c$. Where's the light/radiation?
$\Omega_r$, the energy density contribution of radiation, is $\approx 0.00001$. 
