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I have some questions about this topic. Within 1 kpc of the sun, the mass-to-light ratio is 4 M/L - so for every photon emitted, we have 4 matter-particles.

Now the ratio for the Milky Way is 10 M/L, which is 2.5 times bigger. In my book they say that this means that there's 2.5 times more matter, which we do not see (i.e. dark matter). My question is: Why is that? Who says that the 2.5 times more matter isn't just dead stars?

Question #2: I have the following expression for the intensity ofa spiralling galaxy: I(R) = K*exp(-R/R_s), where R_s is a few parsecs. If I(R) is constant, then is the mass constant too?