[AnimalMuppet]

Not the same. MOND adds a parameter - the non-newtonianness which is purely a function of the masses and the distances. DM lets you add a new parameter (the DM density) at each point in space. That's effectively an infinite number of parameters, whereas MOND has very few.

[mr_mitm]

> DM lets you add a new parameter (the DM density) at each point in space. That's effectively an infinite number of parameters, whereas MOND has very few.

No one is doing that, though. What cosmologists do is parameterize the statistics of the DM distribution. That's one or two parameters. Then we compare observations to simulations to determine how likely the observed distribution is given the statistical properties. For example, a few galaxies with almost no dark matter would be expected due to the dynamics of clusters and galaxies. You could in principle calculate how often that should be the case, and if we were to observe it much more often than we should then there would be a problem with DM. No one is suggesting that the DM distribution can assume any arbitrary shape.

[AnimalMuppet]

That's at the universe-sized level. At the galaxy level, as you state, we say "oh, that galaxy has almost no dark matter". That's a per-galaxy parameter. At the Bullet Cluster, we say "the dark matter must be here and here". That's a point-by-point distribution.

[isthatafact]

> "That's a per-galaxy parameter."

No, variation in galaxy properties is an output, not an input, of the model.

You could decide to quantify and catalog different galaxies with one or more parameters that describe their properties. You could then compare whether that catalog is statistically consistent with the output of the model (and must take into account all uncertainties in the model and the observations).

By analogy, you can measure that different people have different heights, but it does not mean that the specific height of each individual person is a unique input parameter in any fundamental model of biology.

[AnimalMuppet]

Let me change your analogy. You take each person, and measure their height. You also "measure" how tall they "should be". You then show that the differences between their actual height and the height they should have had fits a model. That's nice, but for each person, you still assigned a value for the difference between how tall they are and how tall they should have been.

That's what I mean by "it's a per-galaxy parameter". For each galaxy, to explain the behavior of that galaxy, you're saying "it must have X amount of dark matter".

[isthatafact]

There is no DNA for galaxies, so how could you know what the properties of a particular galaxy "should be"?

The focus on "per-galaxy parameters" is like expecting to be able to predict how tall Tom Cruise should be after reading a textbook on the theory of evolution.

[throwawaymaths]

Ok so you are admitting that just as evolution is a poor model for human height, DM is a poor model for galaxy rotation curves.