# Why can't “missing mass” (=dark matter) be photons?

After a star lives and dies, I assume virtually all of its mass would be photons. If enough stars have already lived and died, couldn’t there be enough photon energy out there to account for all the "missing mass" (=dark matter) in the universe?

And if there were enough photons to account for all the missing mass, what would it look like to us?

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Only a tiny fraction of a star's mass ever converts into photons. –  David Z Nov 29 '12 at 7:28

As a general rule, zero mass particles which travel with the velocity of light are not good for dark matter, because dark matter concentrates around gravitational attractors. It has to be particles with some mass that can be at rest in order to stay around a galactic center from the beginning . In addition they have to be controlled by weak interactions, if they decay, because the dark matter halo is stable for long periods.

Maybe I should add that very cool and slow photons from the beginning of the formation of the observed universe exist and have been detected as Comsmic Microwave Background radiation , very low frequency photons, uniformly distributed in the cosmos.

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Thanks for your answer. If there were a sea of photons, would they not also concentrate around gravitational attractors? Wouldn't there be more photons in the potential energy well of the gravitational attractor than there would be in flat space? Wouldn't this situation be stable? Are particles with mass really the only candidates? –  Tom Fangrow Nov 29 '12 at 6:52
Aside from the unstable orbit at $r = 3M$ you can't bind photons in a gravitational well. The reason is simply that they always travel at $c$, while particles with mass can take any speed. That's why planets can have stable orbits round the Sun. There is a single distance from a mass where the velocity of light matches it's orbital velocity, but even this orbit is unstable so you'd never find any significant concentration of photons there. –  John Rennie Nov 29 '12 at 7:15
As John says; a particle has to have some mass to stay at an orbit.CMB does not concentrate around galaxies, and that is the coldest photons we have observed, because they still travel at the velocity of light. –  anna v Nov 29 '12 at 7:31

There is a simple argument why photons emitted by stars can't be dark matter, and that's because there is about ten times more dark matter than normal matter. If all the stars created at the Big Bang had turned into photons there still wouldn't be enough of them.

You might argue that maybe more normal matter than we think was created during the Big Bang, but the theory of Big Bang Nucleosynthesis places a limit on how much normal matter was created, and this limit is four times smaller than the amount of dark matter. The dark matter has to be something odd.

If you're interested in more info this paper is a good review, though harder going than the answers here!

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Thanks for this, I'll check it out. –  Tom Fangrow Nov 29 '12 at 7:31

Photons don't have mass. So your assumption's incorrect, although I don't know how much of a (say) main sequence star's mass gets converted into photons over its lifespan.

Photon's can't account for, say, dark matter, because dark matter has mass.

Thermal energy (in the vacuum) is comprised of photons, which can spontaneously form particle-antiparticle pairs. Usually these quickly annihilate, so this is also not a good source of mass.

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-1. Photons don't have mass, but they do have energy. It is fully possible that during a star's lifespan, some amount of it's mass is converted into energy (i.e., photons). –  Kitchi Nov 29 '12 at 6:32
The assumption was that all a star's mass is converted to energy in the form of photons. Humor me for a moment. You say dark matter has mass. How do you know? Is it because it affects gravity? Do we know for certain that photons do not affect gravity? –  Tom Fangrow Nov 29 '12 at 6:44
Photons do affect gravity, but that's because gravity is affected by energy, not mass. –  David Z Nov 29 '12 at 7:28
Photons do affect and are affected by gravity ( gravitational lensing) they cannot be captured by gravitational wells because of the velocity c. See Johhn's comment in my answer. –  anna v Nov 29 '12 at 7:33
@Ryan - yes, in the absence of mass gravity is affected by energy and even unlikely things like pressure. The source of the curvature is the stress-energy tensor (en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor) and this makes no distinction between mass and energy. They are treated as related by the (in)famous $E = mc^2$. –  John Rennie Nov 29 '12 at 10:50

Photons are easily detectable. We can count how many photons are there at any distance of us by just counting the photons reaching us from there. It is impossible that the hidden photons ramble the whole universe but mysteriously avoid us.

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I just want to point out that it seems some people may be conflating 'dark matter' with 'dark energy.' Regular "normal" matter like electrons, neutrons, and the like, are estimated to make up about 5% of the matter/energy density of our universe.

Dark matter, estimated to make up about 25% of the matter/energy density of the universe, is matter that has mass, but the gravitational and other effects of which are not directly visible. Dark matter is somewhat mysterious but could easily be something like exotic particles or oceans of black holes between galaxies.

Dark energy, is the real mystery; it makes up the other (about) 70% of the matter/energy density of the universe needed to explain inflationary cosmology and expansion/acceleration of the universe.

Photons are mass-less particles that embody energy, visible when they strike objects. I think it's an interesting notion that the energy from photons could at least in part constitute some of solution to the "missing" dark energy problem. As has been pointed out, it's difficult to reconcile how so much of the "missing" energy could have come from so little: the 5% ordinary matter creating all that dark energy. But I don't see anything impossible about this idea generally. Perhaps the dark matter also contributes to this somehow. There may even be dark-electromagnetic forces that create dark-photons, this may be seen as extra-dimensional as one poster referenced earlier.

"It’s humbling to think that ordinary matter, including all of the elementary particles we’ve ever detected in laboratory experiments, only makes up about 5% of the energy density of the universe." _Sean Carroll

With so little of what we are used to seeing and interacting with in ordinary meaningful way actually making up what exists, speculation of what else is out there is not only justified but necessary.

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