Well, any other theory would have to explain all of the following better than cold dark matter does:

1. Galactic rotation curves (galaxies spin too fast)

2. Galactic gravitional lensing (galaxies bend too much light)

3. The shape of the cosmic microwave background (it only matches if you have dark matter)

4. The bullet cluster (visible matter collided, but the gravitional lensing kept going)

Non-electromagnetically interacting matter solves all of these, and the same ratio of matter:dark matter solves all of them. That would be one hell of a coincidence.

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XorNot 18 minutes ago | parent | next [-]

It also has to do so without destroying a bunch of other exceptionally well tested theories - it's very easy to explain something if you ignore other requirements, like being compatible with relativity - which has been a big problem for MOND but is no problem at all for dark matter (because it's just extra matter being normal out in space).

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daveguy 2 hours ago | parent | prev [-]

I don't understand why it has to be non-electromagnetically interacting. Can we honestly say we should be able to resolve all the matter in a galaxy with our current sensing technologies? Why can't it just be the equivalent of non-light-emitting asteroids and planets. We can barely detect whether a star has planets in our own galaxy. And according to Wikipedia the amount of matter we would be missing in a sphere with radius from the sun to Neptune is 10^17 kg or approximately a large asteroid. Which seems like a rounding error. But my knowledge of astronomy is definitely minimal.

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dontlikeyoueith an hour ago | parent | next [-]

Do you honestly think you've come up with a clever idea that none of the researchers in the field already considered?

You are describing MACHOS, which was one hypothesis for dark matter. https://en.wikipedia.org/wiki/Massive_compact_halo_object

The thing is, we've looked for them. If they were small (asteroid sized), the amount required would be visible as dark clouds obscuring the objects behind them. If they were large (brown dwarf or bigger), we'd see gravitational lensing from them. We don't really see either of those on the scale required to explain the gravitational effect.

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daveguy an hour ago | parent [-]

shit man. I was asking a question. So much for curiosity at HN.

	▲	auntienomen 28 minutes ago \| parent \| next [-]
		I think it's the "can we honestly say" phrasing that got you flamed here. It reads as "those stupid scientists don't know what they're doing".
	▲	andrewflnr 42 minutes ago \| parent \| prev \| next [-]
		Curiosity is one thing, but in practice "just asking questions about dark matter" is uncomfortably close to pseudoscience and a lot of us are sick of it. It's similar to if you had accidentally said something similar to a young-earth creationist.
	▲	z3c0 37 minutes ago \| parent \| prev [-]
		HN is less hackers than it is pedantic VC lackeys training for their aspirational role as a wealthy-and-opinionated Twitter personality.

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Starlevel004 2 hours ago | parent | prev [-]

> I don't understand why it has to be non-electromagnetically interacting

Because we have zero evidence of it electromagnetically interacting. We have evidence of other particles like this, like neutrinos.

> Why can't it just be the equivalent of non-light-emitting asteroids and planets

Because they've been ruled out: https://en.wikipedia.org/wiki/Massive_compact_halo_object

You are not proposing anything new. Your ideas have been thought of before, and discarded because they lack evidence.

	▲	daveguy an hour ago \| parent [-]
		Hah. I didn't think I was proposing anything new. I was telling you how it sounds to someone who isn't an astronomer.