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I'm trying to get my head around what the adiabaticity of fluctuations in the CMB actually tells us. So far as I understand, it points to dark matter because: For the sheer number of galaxies we observe in the universe to form without dark matter, primordial baryonic density fluctuations would have to be huge. That would leave us with pretty big variations in the CMB in the present day, which we don't observe.

That's distilled from a paragraph of a paper I'm reading. However this paragraph is preceded by an introductory paragraph in which the critical density of the universe is defined, and which states that the baryonic density parameter is small. What is the link between these parameters and the argument about the CMB and dark matter that follows? I suspect the fact that I can't answer that myself means I haven't understood, so any help is much appreciated.

Here is the paper, and the relevant section starts at the end of page 18 and is a few paragraphs long.

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