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Could photons be dark matter?

Hi, I'm not any kind of astronomer, or teacher or anything like that. I'm just curious about something that I've been puzzled over for some time now. I often hear about scientists searching for the elusive answer to the question "what is dark matter". I was wondering if it's possible that dark matter could be light. I know that probably sounds silly, but think of it this way. You don't see light between bodies in space and the sun, yet we know it's there because it strikes the bodies and produces illumination when it is destroyed. We think of space as being empty. What if it's not. What if the universe is completely filled with an 'atmosphere' of light particles? Much like the air we breathe. We can't see it, we can only see how it affects our surroundings. If someone were to take a shot at adding up all the light particles eminating from all the stars and quasars and galaxies.... is there a chance that it could all together add up to the missing dark matter? Or, could all the reactions occurring on all the bodies in space from light particles being destroyed and releasing there energy add up to the repulsive force that has been measured and attributed to dark matter? Thanks for taking the time to read what is probably an amusing question to a well educated mind.

The universe is filled with a 'bath' of light. You might even have heard of it. It's called the Cosmic Microwave Background because now it is easiest to detect in the microwave range of the electromagnetic spectrum. These photons are one of the best pieces of evidence that we have for the hot Big Bang model of the universe. As the universe expands the photons are shifted to longer wavelengths and the 'temperature' of the photons cools. Now we say that they have a temperature of 3 Kelvin (-270 Celsius), because they have the spectrum of a black body at a temperature of 3K (see definition of a black body here.). This means that when the universe was 1000 times smaller than it is now they would have had a temperature of 3000 K. This temperature is the temperature at which electrons will combine with nuclei to form atoms (above it -- or before then in the history of the universe they would not stick together). When atoms first formed at this time the universe first became transparent to radiation, and the CMB photons we see are the photons released at that time.

Since we know that the universe is filled with a bath of light we can figure out how much energy is in it and account for it in the models of the universe. This is done, and the dark matter that we talk about is extra missing mass on top of that.

The "repulsive force" you mention is different again from dark matter. We call it "dark energy" or the "cosmological constant" and it comes from both the observation that the expansion of the universe is speeding up, and also the the total energy density in the universe is larger than what is accounted for by normal matter and dark matter put together.

January 2003, Karen Masters (more by Karen Masters) (Like this Answer)

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