[kelseyfrog]

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.

[btilly]

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.

[kelseyfrog]

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?

[btilly]

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.

[kelseyfrog]

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.

[btilly]

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.