[throwaway613834]

But isn't it a hell of a lot less of a stretch to imagine there are more photons in other parts of the universe than here, than to imagine there is some sort of mysterious dark matter permeating all of the universe? If we're going to be unable to interact with whatever dark matter is to test may hypothesis, we might as well propose the hypothesis that requires the least radical changes to our fundamental theories of physics, no?

[eipipuz]

Assume have an equation like `Energy = 0`, where `Energy = Mass + Photons`. It works great for almost everything, we can get the curvature of space by `f(Energy)` where `f(Photons) = 0`.

However there's this one experiment that says the answer is `Energy = 42`. `f(x)` is still useful, everything else works.

We can just amend the equation to be: `Energy = Mass + Photons + DarkMatter` where `DarkMatter = -42` and `f(DarkMatter) = 0`.

Or we can keep the E=M+P but change the way f works to be: `f(Photons, X) = Y` where for almost any value `f(Photons, X') = 0` and in this one case `f(Photons, X") = 42`.

To me it's clear that the first approach is less complicated. The first approach raises one question, "why -42?" The second approach raises "why does X?" exist and "why 42?"

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By the way, you are mistaken, we do interact with Dark Matter, that's how we detected it (Bullet Cluster). Gravity does interact with it.