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Comment by JackFr | Hacker News Reader

The fundamental difference is that we observe and interact with gases, electricity, heat and sound.

Dark matter is a plug. We observe the mass of galaxies and based on our understanding of gravity the galaxies should fly apart. The most parsimonious explanation is that there is a large amount of matter we don’t/can’t observe.

We don’t observe it and we don’t interact with it. While it’s the best explanation we currently have, it rankles to have to admit that the majority of the universe is made of something literally unobservable to us except through gravitational effects.

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We do observe dark matter though, just not directly through the electromagnetic spectrum. Much as the first extrasolar planets were found through deviations in stars' movement before we first imaged them.

no. We infer dark matter. To use gravitation to say "there might be dark matter" and then later say 'dark matter exists because we see gravitational anomalies' is begging the question.

Everything is infered. That's why pretty much every scientific model eventually gets overturned or at least refined.

no, you can deduce a new observation that is then observed.

In the case of dark matter, this would most convincingly take the form of a multimodal observation. Predict that X particle is DM. It then has interaction Y that isn't gravitational. Then Y, which we were not looking for yet, is found.

> unobservable to us except through gravitational effects

That’s how we observe it. We just don’t know what it is built from.

(On the other hand, does it really have to be built from anything? Like, can’t it be some sort of an “intrinsic” distortion of spacetime that, naturally, has an effect on the “normal” matter in the only way it can do so?)

I think I recall reading that it bothered Newton greatly that gravity wasn't made of anything, but he decided that it didn't matter.

> it didn't matter.

I see what you did here.

I don't understand the sentences that contain "observe and interact".

What does "observe" mean to you? It's a sticky philosophical question, but I think that consistency in definitions is essential.

Why does it “rankle”? Would you expect, a priori, that the universe would be very unlikely to contain large amounts of matter that only interacts gravitationally?

Because a priori we typically assume that we’re not special, so it’s odd to assume that 85% of the matter of the universe is fundamentally unlike all the matter we interact with.

I mean, isn't that the definition of black matter? Matter that has gravity but doesn't interact with normal matter through other forces normally?

Yes, it is, “other stuff” that we can’t explain or model without XX degree polynomials that require hand tuning.

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