Ask HN: Is Dark Matter a theory (or a placeholder for observations)? | Hacker News Reader

Ask HN: Is Dark Matter a theory (or a placeholder for observations)?

7 pointskarmakaze8 comments

There's a thread in the comments for "Fitting an elephant with four non-zero parameters"[0] on whether dark matter is a theory. I'm posting the question separately hoping for a more methodical deconstruction and understanding of what we mean when reading "dark matter" and its merits/evidence.

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

It is one of several (well, at least two) hypotheses proposed as putative explanations for the relevant observations. Hypotheses are more than placeholders for theories; they are at least outlines of them, needing completion and verification.

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.

Relevant xkcd: https://xkcd.com/1758/

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.

Using Wikipedia gives reasonably good overview https://en.wikipedia.org/wiki/Dark_matter

That starts out well but then the language changes in the way I described:

> Dark matter is not known to interact with ordinary baryonic matter and radiation except through gravity,[b] making it difficult to detect in the laboratory.

The properties/interactions of DM are defined by our observations rather than observing that it doesn't interact with baryonic matter. You can see how replacing each mention of dark matter with 'the unknown cause of anomalous gravitational observations' is the same minus the belief that it's a particle. This is why I question its theory'ness.

Theories proceed from observations. Based off the observed anomalies and our knowledge of the Standard Model, we can say a lot about the properties of Dark Matter - if it exists.

Going forward, we have two choices:

1. Develop another explanation for the observed anomalies (the so-called MOND theories). So far, these have all failed. Turns out GR is incredibly difficult to both replicate and extend.

2. Provide observational evidence for Dark Matter. People are working to figure out how to go about doing that. See https://atlas.cern/updates/briefing/probing-dark-matter-higg....

Can the theory of dark matter fail? If we never find evidence of such particles, that only means we haven't found them. But we can never rule out their existence.

It becomes a game of statistics. Once we believe we have the capability for detecting dark matter, then each experiment failing to reveal dark matter works against their existence. Keep in mind we can't rule out the existence of Unicorns, either, but that doesn't mean we seriously believe they exist.

Wikipedia also does a good job explaining where the idea for Dark Matter arose in the first place: https://en.wikipedia.org/wiki/Lambda-CDM_model

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