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I'm no cosmotolicist, so forgive me if I'm completely off the mark here.
A few years back, I read that there is not enough matter in the universe to account for the gravitational forces at play, keeping the stars clumped together as galaxies. Then the other day, I saw a show that I'd seen before about the structure of the solar system. It said that we cannot directly observe the Oort cloud, but we can infer it's existence by the stuff we do see, mainly comets. So there's pretty solid evidence that it exists.

That got me thinking as it did before, that if our local run-of-the-mill star has an Oort cloud, this ginormous cloud of ice n shit, then every other star must have one too. Apparently, this thing is so gigantically huge that it reaches half way to Alpha Centauri, where I suppose, it meets that star's Oort cloud. Just imagine how large this cloud would be for a supergiant or a massive black hole. If that is correct then these clouds must fill the whole galaxy pretty much, I'm not good at maths but that's a lot of mass, right?

So was all this extra mass taken into account?
This might sound silly to physics professionals, but why doesn't that much debris floating around affect observations from here on earth, even just to dim intensity of a star's light, so as to give an incorrect estimate of distance or size?

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  • $\begingroup$ What's a "cosmotolicist"? Do you mean cosmologist? $\endgroup$ – ACuriousMind Sep 1 '17 at 11:48
  • $\begingroup$ @ACuriousMind It was a joke. didn't you see me smirk? :) $\endgroup$ – Fizikly Q-Ryus Sep 1 '17 at 13:00
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We should start with the disclaimer that we don't even know for certain if the Oort cloud exists. But if we accept that it does exist and is well described by our current theories of planetary system formation then its total mass is around five times the mass of the Earth. This is several orders of magnitude too small to account for dark matter.

There is about five times as much dark matter as there is visible matter, so the dark matter associated with the Solar System would be around five times the mass of the Sun or about $10^{31}$ kg. The total mass of the Oort cloud would be around $3 \times 10^{25}$ kg.

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  • $\begingroup$ Wow, - that's a lot of unaccounted for mass! So if you isolate the solar system, does this extra mass still need to be accounted for? Or is it just when you calculate mass for our galaxy or universe as a whole that the problem arises? Excuse my curiosity, it's soooo interesting. $\endgroup$ – Fizikly Q-Ryus Sep 1 '17 at 13:12
  • $\begingroup$ That's a bit of a coincidence. The guy who discovered or postulated the theory of dark matter; his name is Jan Oort! The structure must have been named after him. $\endgroup$ – Fizikly Q-Ryus Sep 1 '17 at 13:21
  • $\begingroup$ @FiziklyQ-Ryus: the dark matter density is too low for it to be observed on the scale of the Solar System e.g. it's density is too low to affect the orbits of planets in any detectable way. It's only when we get to galactic scales i.e. tens of thousands of light years that its effect is big enough to be measured. $\endgroup$ – John Rennie Sep 1 '17 at 13:55

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