the reason this isn't true is that by the hypothesis of dark matter, it follows gravity but not electromagnetism. as such it only fits distributions recoverable from evolving gravity. e.g. if we require a certain distribution today, it fixes the distribution at all other points in time, and we can use light speed delay to look into the past to verify whether the distributions have evolved according to gravity.
All observations of individual galaxies occur at a specific point in time. We can’t use light speed delay to see the evolution of individual galaxies only completely different galaxies at some other point in time. As such each galaxy gets its own value for the amount of dark matter.
At minimum this is a ~200 billion parameter model, and more if you’re looking at smaller structures.
That's equally true of the distribution of baryonic matter. We have to assess each galaxy individually to figure out what it's made of? What a crime against science. Never mind that they're still all made of a small handful of types of parts, which can nevertheless combine to form lots of possible histories and shapes for individual objects. Just like literally everything else in the observable universe. Seriously, what part of this argument is different for computing the amount of visible mass in each galaxy?
The availability to detect visible light from stars or detect that light being blocked by baryonic matter.
With dark matter it’s two steps removed where we’re inferring the behavior of baryonic matter and then inferring the amount of baryonic matter we aren’t observing and then calculating the existence of dark matter to get that behavior after accounting for undetected baryonic matter.
Yeah, that's a pain, but calculating mass from photons is still pretty indirect. More importantly, and independently of "directness", no one pretends that galaxies having different masses introduces two billion parameters into our models of cosmology. Because that's not what a model of cosmology is.
Calculating the percentage of the universe’s observable mass is dark matter adds 200+ billion parameters because the mass fraction of each galaxy varies.
So there’s no simple way to calculate it from say looking at the Milky Way alone and extrapolating from the baryonic mass of the rest of the universe. Trying to approximate things from a representative sample is its own problem.
You're still confusing a physics model with a map of the universe. That said, it's sure a heck of a coincidence that the number they get from adding up estimated dark matter in galaxies lines up with the number they get from other cosmological measurements, isn't it? Almost like galaxy rotation curves aren't the only evidence for dark matter and haven't been for a long time. https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...
Gravitational lensing, velocity dispersions, etc circles back to the total mass of galaxies. So it shows up on many of the ways we calculate the total mass fraction not just rotation anomalies.
Something being consistent with a model is different than something being sufficient evidence on its own to support a model.
If the observed dark matter fractions of all known galaxies were 0% but the CMB was unchanged we wouldn’t assume dark matter exists. Thus your #2 is false. There’s infinite models consistent with any observation so finding something after a model was created for other reasons is useful as validation, but the chain of logic is still dependent on the prior observations not the model.
In a meaningfully different cosmos different observations would have happened and different models would exist. Trying to pick out specific experiments as sufficient on their own glosses over that particular limitation.
> If the observed dark matter fractions of all known galaxies were 0% but the CMB was unchanged we wouldn’t assume dark matter exists.
No, astrophysicists would eventually figure out something was up when they couldn't replicate the actual spectrum with dark-matter-free simulations. Why would you assume otherwise? Unless you want to dig into the assumptions of the scenario, in which case you're probably proposing a self-inconsistent universe so of course you can draw whatever conclusions you want from it.
> There’s infinite models consistent with any observation...
You can't actually believe this and still believe in science. If observations don't constrain models, then there is no point in observing. And in the long run, there's asymptotically no difference between "prior observations" and later observations. They're just observations that all go into the same model-constraining mill. Scientists are not fools, and are capable of realizing when an initial observation put them on a wrong trail.
You're still barely touching the real point. This all just sounds like rationalizations to avoid the fact that dark matter, for now at least, and for all that it genuinely sucks, is the Occam's razor explanation for the full suite of observations. Why is this so hard to accept?
