In particular I don't like the language used to talk about dark matter where it presupposes its existence (implicitly or explicitly as a particle). Here's what NASA says (from a search summary):
> Scientists have overwhelming indirect evidence for dark matter. However, its nature remains a mystery. Alternative theoretical explanations for the effects of dark matter, such as modifications to the theory of gravity, don't fit with observational evidence.
I interpret that to actually be saying that we have evidence for a cause (of gravitational warping of space) not explained by known matter, and the mysterious properties of the hypothetical particle are defined by our observations.
However it reads as if we have evidence that the cause of the anomalous observations is caused by a particle that we have yet to discover. What I believe is true in the statement is that we have evidence for 'a cause' of gravitational warping that's not explained by visible matter. This is where I see 'dark matter' as a placeholder for the cause rather than being a theory. The theory part is the hypothesis that it's a particle.
In that comment thread there was a link to a video in support of DM[1] but there's a later video by the same physicist that says "Dark Matter is not a theory."[2]
What I'd like to hear are actual arguments/evidence for dark matter as a particle and not only as the supposed cause of observations. What constitutes a theory? Is it falsifiable--what predictions does it make? Remember that I mean dark matter as a particle, not merely the placeholder cause of observations.
[0] https://news.ycombinator.com/item?id=40963179
[1] https://www.youtube.com/live/8rok8E_tz8k
[2] https://www.youtube.com/watch?v=PbmJkMhmrVI
Proponents of dark matter have not defined it in terms of the observations it is supposed to explain; they have set out the known constraints on what it would be like, if it exists, but these constraints are not tight enough to amount to a definition.
The language you quote disapprovingly is just a person acting like one, insinuating that the hypothesis they find more desirable or plausible is as good as settled, when it really isn't.
Dark matter really is a theory with falsifiable data. I'm not sure of the exact arguments, but I believe the evidence available (such as the evolution of galaxies) is only consistent with the existence of actual dark matter particles, and not with some alternative explanation of gravity.
It's not as simple as saying the mysterious properties of the hypothetical particles are defined solely by our observation, because simply assuming the existence of these massive particles explains all the experimental data within our current framework of gravity, while no other theory has successfully explained the data. We don't need to invent any mysterious new properties beyond the properties that we know massive things have.
So yes, maybe we will one day discover that dark matter isn't real. But by Occam's razor, it seems very likely that it's real, and nobody has a better idea. If it is the only theory that fits all the data, there doesn't seem to be a good reason to reject it.