alt Hacker NewsMy lay question is would the discovery of a fuckton of cold objects like dust or rogue planets or cold sub brown dwarfs clear up the dark matter issue regarding the spin of galaxies?
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Probably not. It requires a lot of mass to make galaxies make sense. And we can compute that the extra matter is usually distributed in a halo around the outer edge of the galaxy. The density is such that if it were any kind of normal matter, we should be able to see it in at least one galaxy.
All of our observation tells us that whatever dark matter is, it doesn't interact with the electromagnetic field. That is, it does not interact with light of any frequency, so is completely undetectable to us. As far as we can tell, dark matter appears to not interact with anything other than gravity, which is pretty weird. We think that it doesn't even interact with itself; two dark matter particles should just pass through each other.
We know a lot about what dark matter isn't, but we're still pretty clueless on what it is
That’s one of the first explanations that was considered. It can’t be dust or gas, because those would be backlit by the stars behind them and thus be visible. Larger objects are called MACHO [0] and have been ruled out by observational studies as well. As explained in [1], baryonic dark matter is also contradicted by CMB anisotropy analysis and nucleosynthesis theory.
[0] https://en.wikipedia.org/wiki/Massive_compact_halo_object
[1] https://en.wikipedia.org/wiki/Dark_matter#Baryonic_matter
That was one of the early ideas, yeah.
No. There's too much of it for that.
If this missing mass were in the form of dust the entire night sky (relatively speaking) should be full of dust reflecting light and radiating infrared due to being lit by starlight for billions of years. But we don't.
If the missing mass were in rogue planets, brown dwarfs, or even cold dwarf stars there would need to be so very many of them that we should be detecting them by the millions with our current telescopes as they pass between us and distant stars. But we don't.
The better our telescopes get the more and more certain we are that the missing mass is not normal matter as we know it. We are getting really good at spotting dim objects (or their side-effects) even when hidden by the glare of stars. Normal matter but not radiating much energy just can't hide from us well enough to account for the missing mass.