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Comment by santoshalper | Hacker News Reader

Actually, the hard thing about dark matter is that the arguments for it and the arguments against it are both relatively compelling. No matter how this shakes out, we're going to have a very different view of the universe when we finally understand what is really happening.

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There aren't really any satisfactory alternatives to dark matter. The question is what exactly is it, and how did it get there.

> There aren't really any satisfactory alternatives to dark matter (given current models)

Dark Matter is a fudge factor used to account for quite good, but still incomplete physical modeling.

“Fudge factor” massively undersells it. For instance, measurements from the bullet cluster are quite compelling that there is significant mass in a galaxy that we don’t directly measure. That’s the definition of dark matter.

>the arguments for it

We don't understand why our numbers don't add up so we made up a number to explain it is "compelling" to you?

Dark matter has always reeked of "ether" to me. I looked into some of the alt science on this a long time ago. There is a compelling case that our model of gravity is wrong. Fortunately gravity is such a weak force that at small ie solar system scales it doesn't matter. Which is why we can launch satellites around the system. However the failure shows at large scale like galaxies.

Dark matter isn't just a made-up number. It's an observed phenomenon that MOND and other modified-gravity theories cannot explain.

It is an observable fact that dark matter is a distinct substance from baryonic matter. Modified gravity is thoroughly disproven by the many galaxies that have been discovered that have been separated from their dark matter[1], or seem to have never had any in the first place [2]. A galaxy can't be separated from, or be lacking in, something that doesn't exist.

Any attempt to dismiss dark matter entirely without addressing this highly conclusive evidence is not valid.

1. https://www.wikiwand.com/en/Bullet_Cluster

2. https://www.inverse.com/science/dark-matter-in-galaxies

Its been a very long time since I looked at any of these stuff and I'm certainly not a subject matter expert. I just remember reading about it and the entire idea of dark matter and dark energy came from the fact that the math models they had at the time didn't add up. They needed a way to rectify the math and they did it by saying well if we put this LITERALLY made up number in the equations everything balances out. Then they made up the explanation as to why this number needed to exist as "dark matter/energy" There have been mountains of research made trying to prove this since then that muddy the water of how flimsy the origin was.

Dark matter has always smelled of phlogistons to me. It's this mystery substance that solves all of our problems, and doesn't make any predictions. I have no idea what a more accurate explanation for the observations would look like, but dark matter can't be it.

It does seem like that, and so there have been many attempts to revise the math to make it fit all known observations. But nobody has been successful, and so dark matter remains, at the very least, by far the best explanation we have. It fits the data.

> It fits the data.

To use an analogy from the world of finance, that sounds like building a Balance Sheet model that "plugs" Retained Earnings so that Assets and Liabilities always balance. Of course it fits the data, its very existence is to explain away the part of the model that doesn't fit the data

Dark matter is not just an imaginary number that fills some holes in math. It is a thing that has been observed. Galaxies can have it or not have it. Galaxies can be separated from it in collisions. You can't be separated from something that doesn't exist.

This article has introduced me to "Modified Newtonian dynamics", which is a field of study I didn't know existed. The premise is pretty obvious now: Newton was pretty cool and all, but he wasn't right about high acceleration environments. Why then should we presume that he was be right about low acceleration environments, when the observations don't fit the predictions? The fact that nobody has come up with an answer doesn't mean there isn't any, and at least these people are actively trying to come up with one. I understand why the author appears to be very frustrated -- I'd be frothing-at-my-mouth angry in his position.

The thing is, it's an extremely simple mechanism which solves all our problems at once. Everything that's currently explained by that of one extra particle, would need separate explanations without dark matter; general relativity + dark matter fits all the data we have. It's not just about galaxy rotation speeds, it's about observations of gravitational mass (as measured by lensing) moving independently from the visible mass, it explains structure formation in the early universe, and other stuff.

It's such a simple explanation which explains so many otherwise unexplained things that I feel we should have pretty strong priors that it's true.

It does make the prediction that there's a lot of detectable massive particles out there. That's not a prediction which is easy to test, but to say there are no predictions isn't right.

My point is that the same could be said about phlogistons back in the day. There were a lot of completely different advances in the understanding of physics and chemistry needed to explain combustion and oxidation, and it's a lot more complex than just saying "because phlogistons". It was an extremely simple mechanism which solved all the problems at once, and fit all the data available at the time.

And it doesn't explain anything! We have this entirely new substance and we don't know what it is, what it's made of, where it came from, or why we only have circumstantial evidence for its presence.

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