[voice_of_reason]

"It might sound like a story from a parallel universe – but it’s true. The Bullet Cluster isn’t the incontrovertible evidence for particle dark matter that you have been told it is. It’s possible to explain the Bullet Cluster with models of modified gravity. And it’s difficult to explain it with particle dark matter"

https://backreaction.blogspot.com/2017/01/the-bullet-cluster...

[perl4ever]

From one of the comments:

"As soon as you start adding extra fields that couple to the gravitational field, you're no longer modifying gravity. You're doing something that is much closer to what dark matter is doing, with the only difference being that you're invoking an additional particle-free field rather than a field that does have particles. Which is frankly just weird because any field should be quantizable and thus any field should have something that looks like a particle, in principle. Your proposal would basically be to throw quantum mechanics out in order to explain the bullet cluster, which is frankly a lot more problematic than simply adding an extra field to the already existing ones."

If I understand the gist of that, the whole argument for modified gravity being somehow simpler seems to go out the window. The author responded:

"That's how it's called. Don't blame me for the terminology. Sure, the field should have particles if you quantize it, but at low energies the classical mean-field approximation should be good."

It sounds like the Occam's Razor argument about modified gravity being better because it's just a revision of universal laws was conceded somewhere along the line, so why even bother?

Edit: From my lay perspective, I imagine it like this: we see dinosaur footprints appearing for no apparent reason. Dark matter theorists say it's invisible dinosaurs. Modified gravity theorists say (I thought) that the laws of gravity can be adjusted to explain it as a "natural" phenomenon. But now it sounds like they are saying there is a continuous field that happens to have significant amplitudes in the places that we would otherwise appear to have quantized invisible dinosaurs. But how does that explain at all the reason for the field having an amplitude here and not there? Why is it more pleasing or likely than distinct individual dinosaurs? It all seems like a bait and switch so ridiculous I feel like I must be misunderstanding grossly.

[acqq]

Still:

https://medium.com/starts-with-a-bang/only-dark-matter-and-n...

"Modified gravity cannot successfully predict the large-scale structure of the Universe the way that a Universe full of dark matter can. Period. And until it can, it’s not worth paying any mind to as a serious competitor. You cannot ignore physical cosmology in your attempts to decipher the cosmos, and the predictions of large-scale structure, the microwave background, the light elements, and the bending of starlight are some of the most basic and important predictions that come out of physical cosmology. "

[voice_of_reason]

And the quote war goes on :)

Mr Siegel asserts that dark matter hypothesis is better than any kind of MOND (and, may I add, his opinion is expressed in over-confident and even dogmatic way) because it can "explain" structure of the Universe. However, people in comments doubt that LCDM is particularly good in this area:

"The problem is that dark matter can’t predict the structure of the universe all that well either. Making those dark matter simulations work requires introducing all sorts of epicycles, kludgy mechanisms to make the calculations work out properly. I’m not a big MOND fan, but MOND seems to solve certain problems fairly well, and the galaxy rotation problem is one of them."

https://medium.com/@kaleberg7/the-problem-is-that-dark-matte...

[acqq]

I don’t see any valid new argument in the qouted comment, yes, the only strength of MOND are galaxy rotations, as mentioned by Ethan, no, no epycycles are actually present in lambda CDM.

https://en.m.wikipedia.org/wiki/Lambda-CDM_model