Neptune was invented when the orbit of Uranus didn't match our theory of gravity.

Vulcan was 'invented' when the orbit of Mercury didn't match our theory of [non-relativistic] gravity.

Then recanted when we improved our knowledge of gravitation.

https://en.m.wikipedia.org/wiki/Vulcan_(hypothetical_planet)

What's the moral of the story?

Sometimes proposed invisible body of mass turns out to really exist, sometimes it is discarded as no longer necessary.

Exactly the point I wanted to make to OP - who was skeptical of dark matter because we didn't throw out General Relatively, one of the most tested and successful theories of all time, when some aberrations were noticed.

The thing is that we're pretty sure our theories are right, but we're still missing a bunch of stuff. We've named that stuff "dark matter". There's really not much to be skeptical about as far as I know.

I have always been a bit skeptical about it being some elusive magical new particle or somesuch though.

    we're pretty sure our theories are right

People are always so sure.

They didn't say "Well, the geocentric model is probably wrong but we'll use it for now" either.

But so far we don't have experiments proving it's wrong, I think that's the crux of the argument ?

I'm certainly not a physicist.

I stopped my physics at a BS. But if I understand correctly, we do have observations that don't match our theory. They would match if there was matter that we can't see. "Dark matter" is the hypothesized missing matter. This lets us keep our theories as they are.

But instead, you could regard the observations as "experiments proving our theories wrong".

There has been work done toward re-working our current laws to fit observation without dark matter, but I think it just breaks way more stuff than it solves. The new model ends up being a worse fit for pretty much everything else but the galaxy rotation thing.

We already have weakly interacting particles like the neutrino, which don't interact with matter outside of the weak force, making them rather difficult to detect, it's not really that far out of the realm of possibility to have other particles that only interact via the gravitational force.

> "I think it just breaks way more stuff than it solves. The new model ends up being a worse fit for pretty much everything else but the galaxy rotation thing."

This is often mentioned as a reason not to spend time with alternative models. However, breaking stuff is sometimes necessary if we are to get out of the local maximum of theory usefulness. The Kopernik model was much worse fit for the observations of planet motion that the Ptolemy's model, but it did have other virtues, like simplicity. The broken parts got fixed later with development of mechanics and perturbation theory.

That is a given. Dark matter isn't really that clean either, it breaks what we know about the standard model and known particles, something that the folks at CERN keep confirming as they break in to new and uncharted areas of what we can observe experiment. Your statement is less enlightening as it is simply stating the inaccurate nature of a model.

You're not wrong. "Dark Matter" is just a name we've given to whatever it is that causes these things we can't otherwise explain.

It does have some properties that we've managed to figure out, and they do look matter-ish.

You should give the book "We Have No Idea"[0] a try. It's the author of PhD Comics. It's first chapter explains pretty well why we know dark matter exists.

[0] https://www.amazon.com/gp/aw/d/0735211515/

What's different about it compared to other scientific theories?

They usually do not just fill a hole in a theory but make a ton of new predictions that then can be verified.

Conjecting that the earth is round did not just explain why ships disappear behind the horizon. It also predicted that you can get back to point A by always moving away from it and many other phenomena.

Since it was first used to explain galactic rotation curves, predictions of seeing the signature of dark matter, in the CMB, in gravitational lensing observations, in the structure of galactic clusters, the expansion of the universe, etc have all held up.

And these not only are consistent with the existence of dark matter, but roughly indicate the same amount of it as a fraction of mass in the universe.

So I disagree strongly that the existence of dark matter hasn't led to other predictions. Indeed, the number of different phenomena that lead to it is one of those things that have made alternative theories so difficult. There are a ton of alternative gravitational theories that can explain one or two of the above phenomena, but trying to match them all (and not contradict other observations), seems to be somewhere between difficult and impossible.

That sounds interesting. Do you have links to some of those predictions and how measurements held up?

There are lots of verifiable dark matter predictions, just none of them have been verified yet. The article mentions some of the predictions around WIMPs. It's not like string theory where there's no way to disprove it.

I like to repeat myself on this, but proposing "invisible" matter has historically been a very successful strategy in fundamental physics. Simple laws have a tendency to stay simple.

Good to see that the classic "middlebrow dismissal" is still alive and well here on HN.

The fact is that dark matter isn't just some half-baked theory that astrophysicists pulled out of their ass. Nor does it represent some lame attempt to just sweep complexity under the rug as if astronomers said "I dunno, probably magic", shrugged, and moved on.

Dark matter is a very specific and constrained theory that has withstood the test of decades of observational tests where all other theories have fallen by the wayside. Astrophysicists and cosmologists did not set out trying to force the theory of dark matter on the world, they have tried repeatedly to prove that other explanations could account for observational evidence, but none did. Instead they've had the reality of dark matter thrust upon them.

And it's not such a crazy theory either. We know that there are other kinds of weakly interacting massive particles such as neutrinos that exist in the Universe. And we know that our understanding of particle physics, the standard model, is incomplete. It is not a great leap to posit that potentially there are other types of weakly interacting massive particles that we have yet to discover. We know that neutrinos and "hot dark matter" does not make up the majority of the mass of dark matter in our Universe through several lines of observations. We know that baryonic (atomic) matter is also not dark matter as well. Only "cold" (sub-relativistic speeds) WIMPs fit the evidence for dark matter. The evidence gathered from galactic motion studies, gravitational lensing observations, simulations of the evolution of the large scale structure of the Universe, measurements of the cosmic microwave background, and many other experiments. Nothing fits all the data except the cold dark matter theory. That's why scientists believe it.

The fact that you are skeptical about it is irrelevant. Educate yourself on the evidence and science of it first, and then you can talk.

That is kinda the hole point.

I see what you did there.

I'm missing it...

Exactly.

Scientists: Listen people! We have created models that explain why stuff does what it does! They are so complicated almost nobody can understand them. They involve 4 dimensions, a zoo of particles, randomness in everything and no reality independent of the observer. And they conflict. But at least they explain why stuff does what it does!

World: But they make the wrong predictions. Stuff does not behave like that.

Scientist: Oh, no problem. Just imagine there is an invisible force out there that moves stuff around. Imagine most of our universe is made of that invisible stuff. Now back to our awesome models!

This doesn't strike me as an especially powerful critique, in spite of your provocative language.

We have well understood mathematical models which correctly and accurately predict all physical behavior relevant to our everyday lives, and, further, correctly and accurately predict the majority of our observations of the universe around us, period. As tools for understanding our world, they are incredible achievements.

In spite of us knowing these models to be fundamentally incomplete, we have yet (to my understanding) to perform an experiment on Earth which contradicts them. More to the point, the theory which supersedes them will, by virtue of needing to replicate all experimental results so far, will likely be even more esoteric, complicated, wild, baffling, and impenetrable (and--with any luck--elegant, inspiring, and beautiful).

He didn't say it very well, but I think he was getting at the Occams' Razor argument - this seems like an unnecessarily complex set of theories to explain the observed phenomena. (Not that I agree, but I think you missed his point)

IMO, Occam's Razor is only meaningful when there are two competing hypothesis, one of which is markedly simpler than the other. Mentioning it in a context of "surely it can't be this complicated" without pointing towards a simpler alternative doesn't strike me as very useful. I know you're not the person who made the original argument, just chiming in on the discussion.

    accurately predict all physical behavior

Now you made me curious. What is the non-physical (metaphysical?) behaviour that we cannot predict?

We cannot, as an example, currently predict the zeroes of the Riemann zeta function.

What if we make a robot that dances those zeroes. Then we have unpredictable physical behavior.

The robot would have to predict those zeroes to 'dance them'.

Richard Feynman on observations not matching strong theories:

https://www.youtube.com/watch?v=b9F8Wn4vf5Y&t=4m44s

I like Quantised Inertia[1] as an alternative to Dark Matter or MOND. The theory makes some testable predictions, matches existing data (for instance the flyby anomalies, EM drive)[2] and requires no tweaking or "fudging factors".

It's still fairly immature, so still needs work - it violates the equivalence principle and relies on Unruh radiation which hasn't conclusively been observed.

[1] http://physicsfromtheedge.blogspot.co.uk/2014/01/mihsc-101.h... [2] http://physicsfromtheedge.blogspot.co.uk/2016/09/mihscqi-vs-...

I feel like a better reference would be one or two of the journal articles the guy published...not having a physics background to judge the science on its own merit, I'd rather not read some guy's blog post.

Yes I concur. I thought about admitting that i'm only referencing the author's blog posts was a bit "narrow", but it's also probably the best source for authoritative information on the subject. Literally "from the horses mouth".

There are the arxiv papers and some other reports (that all tend to link back to either the blog or arxiv) as well. Like you, I don't have a strong physics background, so find the published papers a challenging read.

https://arxiv.org/pdf/1604.03449v1.pdf