Could photons be dark matter?

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.
367 days ago

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