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djamp42 t1_j13jmhw wrote

Because we can only see so far in the universe, how can we be so sure all the matter that is missing isn't just out of view from us?

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Subject_Meat5314 t1_j14bmjh wrote

The only reason we need dark matter to exist is to explain the things we can see. For galaxies to work they way they work, given current understanding of how gravity works, they need more mass. That mass has to be somewhere in the galaxy. And there are plenty, as in trillions, of galaxies that are entirely contained in what we call the observable universe.

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pressedbread t1_j17wvx1 wrote

>And there are plenty, as in trillions, of galaxies that are entirely contained in what we call the observable universe.

Might not be that much. It seems like a lot because billions or trillions is a large number, but it might be a lot bigger

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Bascna t1_j16awyo wrote

Because the gravitational effects of dark matter can be seen more locally.

For example, the rotational velocities of stars within spinning galaxies don't match the gravitational force that should be generated by the visible matter within those galaxies. To produce those velocities there must be a lot of unseen mass within those galaxies. Mass that is beyond the visible universe wouldn't increase the centripetal force within those galaxies.

Similarly we need unseen mass to account for the amount of curvature that we observe in instances of gravitational lensing. Such mass would need to be between us and the light sources, not beyond those light sources.

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s1ngular1ty2 t1_j14m5yh wrote

The matter that is missing is in the part of the universe we can see because we see it's gravity.

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MisterET t1_j141eo1 wrote

Because we can see tens of light years in every direction. And the matter we see doesn't match with our observations.

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marketrent OP t1_j13i32q wrote

Becky Ferreira, 13 December 2022, Motherboard (Vice Media)

Excerpt:

>One possible method to search for dark matter is to capture “antinuclei,” which are the antimatter versions of the nuclei found in normal atoms. Antinuclei might be generated by interactions between dark matter particles, distinguishing them as a potential window into the longstanding question the nature of this elusive material.

>However, most antinuclei are forged in the dense messy region near the center of our galaxy, tens of thousands of light years away from Earth, so it’s not clear how many of these messengers can reach us across that vast distance.

>Now, scientists have used the ALICE detector at the Large Hadron Collider, the biggest particle accelerator on Earth, to estimate the “transparency” of our galaxy to helium antinuclei, a measurement that makes it possible to estimate how far these particles can travel before they encounter regular matter and disappear.

> 

>The results revealed that antinuclei may indeed voyage across the Milky Way to reach Earth, making them “a very promising channel for the discovery of dark matter” now that we know we can likely detect them here, according to a study published on Monday [Dec. 12] in Nature Physics.

>“Antinuclei don’t travel straight through the galaxy because they are charged and there are magnetic fields” in the Milky Way, said Maximiliano Puccio, a member of the ALICE collaboration and a co-author of the new study, in a call with Motherboard.

>“This means they have a very contorted path of coming to the Earth that is much longer than the linear distance from the center of the galaxy.”

>“When we put all the ingredients together” at CERN “and we saw that half of the [antinuclei] survive, that was quite something,” he added, noting that the finding suggests that these strange particles can wind up around Earth.

Nature Physics, 2022. DOI 10.1038/s41567-022-01804-8

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toodroot t1_j15nahb wrote

What a confusing article! The actual paper makes it clear that ALICE was used to measure the "inelastic interaction cross section" of helium antinuclei.

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Revolutionary_Eye887 t1_j17fcrb wrote

Doesn’t everything in space travel thousands of light years?

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OSUfan88 t1_j19i5xm wrote

Not things that are closer than that distance.

The point was, a vast majority of the dark matter in the Milk Way is generated near the center of the galaxy, which is thousands of lightyears away.

Since antimatter and matter will immediately annihilate with any contact, it's a pretty wild thought that a particle could travel that distance, and not come in contact with anything. Even "empty space" is filled with particles. In our solar system, there's about a thousand atoms/cubic meter of "empty" space. Some areas in the galaxy will be less dense than this, while others much more dense (especially when you approach the center of the Galaxy).

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Bean-Swellington t1_j17fxp4 wrote

Is that something we should be concerned about? It sounds like a bad thing

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rocketsocks t1_j1jdsct wrote

Not particularly. Anti-matter may seem exotic but it's just a part of regular life. Every few seconds a potassium-40 atom in your body will decay in a way that produces a positron (anti-electron) which annihilates with an electron and releases gamma rays. That's one among many common anti-matter reactions that occur regularly around us and within us. High energy reactions are just part of our world, and we live with them. The amount of anti-matter coming from distant galaxies is tiny and not something to worry about on a personal level.

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andimaniax t1_j188sl4 wrote

If it’s antimatter, does that mean it doesn’t matter?

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