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Comment by JackFr | Hacker News Reader

I am not a physics expert, but isn't dark matter simply a plug because mass of galaxies and large scale astronomical structures calculated from light and other electromagnetic radiation, doesn't agree with mass calculated by observing they're gravitational interaction? That is, we assume that both models (radiation & gravity) are correct, and that 90+% of the universe is made up of matter which has never been directly observed and is radically different than any matter we observe directly.

The whole thing seems ludicrous -- but for the problem that no better explanation has yet been put forth. One gets the feeling though that one day in the future 'dark matter' is gonna look a lot like 'epicycles'.

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It's not just galaxy rotation. That's how people first noticed it. Since then there's stuff like this: http://en.wikipedia.org/wiki/Bullet_Cluster

Also there's apparently information about the universe's composition to be gleaned from the CMB anisotropies; its all somewhat beyond me, but that curve certainty seems to fit extremely well, I'll give them that.

http://www.astro.ucla.edu/~wright/CMB-DT.html

The combination of all this stuff is apparently pretty convincing.

EDIT: I see that your comment is actually referring to the bullet cluster more than the rotation curves. It's all pretty tenuous in the grand scheme of things, I agree, but then so much of cosmology is. I guess they have the benefit of the string people next door making them look reasonable by comparison.

> isn't dark matter simply a plug because mass of galaxies and large scale astronomical structures calculated from light and other electromagnetic radiation, doesn't agree with mass calculated by observing they're gravitational interaction?

One might more generously say that dark matter is a proposed explanation for many measurements deviating from predictions based on visible matter in a way very consistent with the existence of large amounts of non-electromagnetically-interacting matter.

I don't think there's anything fundamentally ridiculous about this. For instance, while you characterise it as 'radically' different to any matter that we observe directly, the neutrino has most of the relevant properties, and e.g. supersymmetry has been noted to include dark matter candidate particles. While one may doubt that supersymmetry is actually correct, I think it demonstrates that the combination of properties (massive + electromagnetically inert + maybe weak force interacting) isn't necessarily a problem.

> future 'dark matter' is gonna look a lot like 'epicycles'.

It wasn't until Kepler proposed elliptical orbits at the beginning of the 17th century that heliocentrism actually made good mathematical sense though - Copernicus' perfectly circular orbits still made use of epicycles to explain deviations between calculation an observation.

From the mid 15th century when Copernicus first proposed heliocentrism, to the mid 17th century when Kepler's theories became widely accepted is a long time though.

I want to believe society has advanced past needing centuries to advance from a possibly superior model that still has some issues, to an obviously superior model, but the way seemingly intelligent people cling dogmatically to things like Pluto being a planet, and defining a kilobyte as 1024 bytes because kibibytes 'looks dumb' gives me pause though.

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