We only know about a small fraction of the matter in the Universe | Hacker News Reader

We only know about a small fraction of the matter in the Universe

bbc.com

56 pointskostyk20 comments

> Zwicky's work was largely forgotten until the 1970s, when astronomer Vera Rubin discovered that nearby galaxies were not spinning in the right way.

What Vera observed was that the stars in distant galaxies were all spinning "together". One would expect some stars in the galaxy to spin at a faster speed than others at varying distances from the center. What she observed is akin to a single "galaxy" bitmap rotating all at once, where the relative positions of the stars in the galaxy "image" being in lockstep with each other.

Dark matter. Hehe.

What I have always found mesmerizing about dark matter, is whether it forms deeply complex and interesting structures like ordinary matter (like galaxies, planetary systems, or "dark life", or even radically different structures that we have no concept of). But we just don't know because it's so hard to detect.

Which makes me think that if it's so utterly non-interacting (at least when it comes to the electromagnetic force), are there yet undiscovered fundamental forces that act on dark matter, or does it only interact with gravity? If only gravity, dark-life will be very very very large in size, given how incredibly weak gravity is.

Gravity also doesn't give you chemistry.

Earth intuitions are not good to apply to the universe. Life on Earth may seem very robust and resilient, but the universe as a whole does not seem friendly to it. And yet, even things as hostile as the surface of the sun is as a verdant garden paradise for life compared to the dark matter universe, because at least on the sun, atoms interact with each other. The dark matter universe looks more like one of those universes where someone tweaked the constants so that stars and planets never form and so nothing but hydrogen ever exists. It doesn't matter how much hydrogen you pour into a universe, if that's all you have, it's not going to be alive.

Perhaps dark life is the origin of monster tales. They only interact with normal matter under really rare circumstances. When they do, they are absolutely horrifying because they are so other than us.

I was just on a similar train of thought: Cthulhu-like monsters made of dark matter, traversing the universe. Wouldn't that be fascinating?

http://www.hplovecraft.com/writings/texts/fiction/fb.aspx

We know that the structures that it forms cannot be too complex because otherwise we would detect them through microlensing. What can happen is that the structure would pass in front of the star and gravitationally lens the light of the background star around it to make it temporarily appear brighter. Surveys looking for these events have found that the vast majority of dark matter cannot be in these very dense structures. So dark matter life seems to be ruled out.

I'm about 1m radius; a bacterium is 1um in radius; our sun is 700,000,000m in radius. So it's nearly a billion times my radius, and something like a thousand trillion times the radius of a bacterium.

I don't believe we can't detect anything but purely dark matter planets passing in front of stars, and thus we can't rule out "life", only "planets".

I also don't believe our theory is sufficient to distinguish "generic cloud of dark matter" and "cloud of dark matter in 'organic chemical' analogs", particularly because of our lack of sensitive gravitational measurements to directly test.

Why is dark matter only found away from Earth?

It is certainly found at Earth too. But since you can only interact with it through gravity and the effect is very very small compared to earths gravitation it is not like you can take a picture...

See the answer here (http://physics.stackexchange.com/questions/161267/distributi...). Currently we can only detect dark matter indirectly by measuring its gravitational effect. As far as I understand, at small scales (e.g. scale of solar system) it's density is very small.

Statistically, all matter on the scales we're used to is only found away from earth since the universe as a whole (including dark matter) is something like 10^30 times less dense than earth.

So if dark matter is found independently of matter* there'd be on the order of 10's of mg of it in the whole earth which would seem impossible to ever measure via gravity.

* Probably a huge if, it's just a thought experiment

As the other answers have said, there is dark matter expected near the Earth, it's just at a very low density. There have been a number of experiments looking for some signature of weak dark matter interactions near the Earth. One of these, DAMA, has claimed a detection of an annually varying signal which would be consistent with the Earth moving with and against an ambient medium of dark matter over the course of a year. However, no other experiments have been able to verify this detection.

That fraction is the most interesting fraction though.

Depending on who you ask :).

It's not really subjective. Since dark matter has so little interaction, it probably has just few basic properties like mass and some other. Regular matter has complex things going on for it. That's why there is life, stars, etc. composed of regular matter and not dark matter.

The NASA image of the bullet cluster shows that light bends not exactly where the two galaxies are colliding.

This is taken as evidence for dark matter (because gravity is not where the visible matter is).

But I would not expect the light to bend where the galaxies are. I would expect light to bend where they were.

That light has been traveling for millennia, and has been bending the entire time, so of course it bends at a distant location from where the galaxies are "now".

So I don't understand how that photo is evidence of dark matter.

That photo shows where the galaxies were then, not now.

Are you talking about the speed of light, that everything we see is in the past?

Because I am not talking about that. So if you mean something else please clarify.

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