MOND only really does well on galactic rotation curves because it has free parameters that are tuned to "predict" the correct answer for galactic rotation curves.

There are galaxies that appear to be free of dark matter and rotate accordingly. How does MOND account for that?

My understanding is that these observations are a fatal blow to any serious MOND models.

But at least the experimental results disproving these incremental fixes should be exactly the kind of thing the next Einstein should need for coming up with an entirely new way of looking at things

you think the deepest mysteries of reality and the universe should just reveal themselves because we have a couple thousand smart people working on it for... 10 years?

so MOND is the new String Theory...

I mean it's easy to say it's fake, but to counter this, why can a particle that only interacts with gravity not exist?

The neutrino is a good example of a particle that almost doesn't exist. They are produced in solar reactions in spectacular amounts. Trillions of them are flitting through you right now as if you don't exist. You'd need a light year block of lead to ensure you could stop one. Mind-boggling amounts of them have to pass through our detectors to see even a single interaction.

Simply put, the particle physics does not have to behave nice so you can sleep well at night.

Dark Unknown Matter would be a better name for lay people to understand what's going on. I'm no cosmologist but isn't it just a placeholder for something that gravity interacts with (and not much else) and we don't know what it currently is. When we discover what it is the name will change.

Or, you know, as aether.

It's a scientific theory. It's the best that we have right now to model the real world and be able to do prediction on its behavior.

Does it seems to be kept together by duct tape? Maybe.

Is it yet useful? Yep.

Will it be discarded if anything more fitting will came up? You can be sure of it.

Yeah until you get to quantum computing and then it seems as if the universe is doing enormously more work than you would think necessary.

Everything we don't understand we conceptualize using the most similar tools which we do have command over.

Dark matter is invisible, but it isn't magic. It is not significantly different from neutrinos. No one seriously denies the existence of neutrinos nowadays, even though they are invisible (i.e. they don't interact electromagnetically).

Dark matter is actually a very parsimonious theory. None of the laws of physics have to change to accommodate it, unlike with MOND. We may not see it, but it has to move around and affect normal matter in predictable patterns consistent with our current understanding of physics. If it doesn't, then the theory is wrong and may need some revision (which may be a dark matter + MOND hybrid).

In parallel with the research that attempts to find the properties of dark matter that best describe our observations is research that attempt to find what other properties it may have. It is a new particle? Can it interact in ways other than gravity? We didn't find anything, but the universe is under no obligation to make things easy for us.

One possible idea called the "nightmare scenario" is that dark matter is made of particles that only interacts gravitationally. It is a perfectly fine theory, maybe the cleanest one, but unfortunately, it would mean that we may never be able to detect these particles because gravity is so weak that the required detectors would be way beyond our technological abilities.

We make a similar guess for the stuff that is in the center of Earth. We measure local gravity and speed of sound velocity, and we guess here is liquid, here is solid, here is this rock, here is this another rock [1]. See for example https://www.livescience.com/64943-nobody-understands-the-gia... nobody has seen them, we guess they are there.

Dark matter is another guess. We guess there is more matter in galaxies than what the telescopes show. We can compare the amount of mater guessed from galaxy rotation with other measurements. In this case they compare it with the gravity between a few galaxies.

Nobody is happy that we don't know what dark mater is. There are a few theories, but none of them has enough experimental support. More lack of confirmed details in https://en.wikipedia.org/wiki/Dark_matter#Composition

[1] I don't know enough about rocks https://xkcd.com/2501/