I don't know why this got downvoted.

The top theory is exactly that - most of the mass in a galaxy is in the form of dark matter of some kind that can't fit our current theories of physics. The second top theory is that gravity behaves differently at long distances than at short distances. The third theory that is proposed every so often is general relativity, but those who know it best universally agree that it is too small an effect by several orders of magnitude. Every other theory that I've encountered is generally labeled a crank theory.

The reality is that we don't actually know what's going on with the galaxy curves. But we do know that it involves physics that we do not yet understand. And the leading two theories involve weird fudge factors.

Because it's a conspiratorial mindset that willfully ignores the long list of other evidence for dark matter in favor of mocking physicists who are doing their best to make sense of confusing observations. That's exactly the kind of thing that downvotes are meant for.

I see zero evidence for any of that.

Science has a long history of proposals based on various fudge factors. Some of those fudge factors disappear when we get better measurements. Some require learning more about the system. For example Newton's theory of sound was consistently wrong until Laplace figured out how adiabatic heating changed things. Some require learning more about the physics. For example Einstein got rid of the need for a fudge factor for figuring out Mercury's orbit. Some require proposing new physics.

When dark matter is called a fudge factor, that is absolutely technically correct. It *IS* a fudge factor. It is a fudge factor that explains a whole lot of stuff, at the cost of requiring something new whose nature we have no real clue about. This isn't conspiracy theory. This is how science works. It is how science is supposed to work.

It's current status is much like that of the neutrino when it was first proposed. A particle we can't think of a way to detect in any way other than the fact that it is required for conservation laws to work out. As it happens, we did eventually figure out how to detect it. But only through discovering then-unknown physics (chain reactions), and building detectors which required a budget that was unthinkable for physics back when the neutrino was proposed in 1930.

If we're going to get technical, then the source of confusion here is that the term "dark matter" is considered a theory by lay people. Whereas scientists view dark matter as a set of outstanding problems in cosmology.

There are many dark matter theories that address these problems, and we can talk specifics about each one and how it addresses each of the dark matter problems.

On the one hand, I mostly agree with you.

On the other hand, squint at pretty much any theory, and you can see a series of related theories. That are each the basic theory, plus another assumption or two about another thing that might be observed. Leading to a cascade of differences that result in distinct theories.

Therefore dark matter can be a theory, and there can also be many theories of dark matter. Just like evolution can be a theory, and there can also be many theories about how exactly evolution progresses. Exactly what we call a "theory" here becomes rather arbitrary.

It's a simple thing to check. If there is a theory of evolution, what's the theory of evolution? Likewise, if there is a theory of dark matter, what is the theory of dark matter?

Darwin's theory of evolution is descent with modification. It requires a method of inheritance, a method of random variation, and the observation that a variation which results in more descendants will grow exponentially relative to the rest of the population, until it dominates.

Examples of specific theories include the Modern Synthesis and Punctuated Equilibrium. But there are more specific theories as well. For example https://www.amazon.com/Ontogeny-Phylogeny-Stephen-Jay-Gould/... lays out evidence that one of the major drivers of evolution is that it is easier to change the relative timing of maturation, but almost impossible to change the sequencing. And we have specific examples of evolution where the method of modification is not mutation. For example the placenta is believed to be the result of viruses transfering a parasite genome into the mammal genoume.

The theory of dark matter is the theory that a large fraction of the matter that makes up galaxies is not visible.

You can get different theories based on how much of it we think that there is, and what we think that it is made of. There isn't a ton of room for how much of it there is - Newton plus gravity curves gives us that. But the what is fascinating. For example you might think that it is mostly things like wandering planets and stars that did not ignite. This gives us the theory of MACHOs. But we've been able to establish that most of it can't be that. We've also managed to establish that not a single thing in our known particle zoo will work. And so it has to be something of a kind we've yet to discover.

And that's where we have the challenge. We start with the idea that there's a lot of stuff that isn't shining stars. That's very reasonable. Then we wind up with the conclusion that our galaxy is mostly stuff of a kind that physics knows absolutely nothing about. That's...substantially more surprising.

The problem of dark matter is, "why does lambda-cdm fail in all these ways that makes it look as if there is a bunch of non-interacting stuff in the universe?" There are specific theories that address the problem, but none is the theory of dark matter.

Likewise, Darwin started off with the problem, "what is the origin of species?" answered by natural selection, and its progeny as you mentioned. Again, I'm being nitpicky specifically because this is a good example of metonymy. It's the metonymy itself that causes this confusion.

You call it nitpicky. I call it a meaningless distinction.

Dark matter is no more or less a theory than Darwin's Descent with Modification.

Darwin's theory needed to be modified extensively to deal with genetics. And modified again to deal with paleontological evidence on how speciation works. And modified again for jumping genes. And modified again to cover things like having a virus create a new clade of Mammalia by hybridizing mammals and a parasite. And yet we still talk about Darwin's theory of Evolution.

You are dismissing criticism lightly by accusing others of being conspiracy theorists, mocking you, and so on. These are classic ways of creating cognitive dissonance and shutting off alternate ideas. Which is the exact OPPOSITE of what Feynman tells you to do in https://calteches.library.caltech.edu/51/2/CargoCult.htm.

The truth is this. Dark matter is observationally the best theory that we've got. However it is deeply unsatisfying. It requires fundamentally new physics about fundamentally new stuff with properties that we have absolutely no clue about. It is literally a theory of, "Insert magic cosmic glue here."

Virtually any idea can be inserted. String theory is popular despite having made a single verifiable prediction in decades of trying. So let's say that dark matter is made of strings!

Maybe if the Everett interpretation were more popular we'd theorize that a proper unified theory will have a small gravitational interaction between quantum superpositions. So what looks like dark matter is really the gravitational interaction with the superpositions of the stars in the galaxy that have been evolving since the early universe. Thanks to the fact that multibody gravity systems are chaotic, every tiny variation grows over time until those superpositions just act like a smooth smear, which we can't directly perceive because of quantum decoherence. Is this a reasonable theory? Don't ask me, I just made it up. But I know that it doesn't require any new kind of matter - it just requires a bit of speculation about interpretations of QM and the nature of quantum gravity. And it would look just like dark matter does.

The truth is that we've got a theory, and we know how to fit the data to it. But that theory doesn't integrate well with all of our other theories. Therefore, no matter how well we've made the facts fit, intellectual integrity requires that we remain open to the idea of being wrong. Not so open that we stop pursuing what *WE* think is right. But open enough to recognize that other people's discomfort with the theory is actually somewhat reasonable. No matter how strongly we might think that it is right.

I'm very open to dark matter alternatives. It sucks, everyone hates it. But there are a vocal segment of people who are unable to accept that it might possibly be true anyway, who believe it's some kind of mass delusion by physicists rather than, as it actually is, the current simplest explanation of a whole lot of observations that look just like matter which is dark. Have you really not seen these people on every thread where dark matter comes up?

Well I'm glad that you acknowledge that it sucks.

I see the people who can't accept that it is plausible as being the mirror image of people who object to calling it a fudge factor that makes our numbers work out.

It really isn't currently an explanation. It is a statement about what the explanation will look like. But the actual explanation at this point requires new physics.

I see why some people would downvote him.

However my expectations of HN are that, total, he shouldn't be downvoted for that comment. And indeed, he currently isn't.

I'm glad his downvoting was temporary.