Hacker News new | past | comments | ask | show | jobs | submit login

> what is dark matter? is it the opposite of matter? how can it be called matter when it doesn't exist?

Dark matter is what we call the discrepancy between our observations of how galaxies and galaxy clusters move, and how our theories tell us they should move based on our best estimates for how much normal matter is in those galaxies.

We call it "dark matter" because our formulas give the right prediction if we add a lot of mass where we see none, that is, if we assume there is some matter there that does not emit or interact with light.

AFAIK, the difference between general relativity with 3 spatial dimensions and one with additional spatial dimensions show up on the small scale, and not large scale like galaxies[1].

> could it not be the space-time curvature from the mass itself but from the encapsulating dark matter around it

If dark matter is indeed some kind of matter, then it would indeed affect the space-time curvature. That's in fact the entire point behind calling it dark _matter_, as mentioned above.

Of course there is the possibility that our theories needs to change. There have been proposed alternatives that does not require additional matter in the form of new particles. The problem is that it's very difficult to get the theories to match non-dark matter observations, all the "normal stuff", as well as dark matter observations.

[1]: https://www.forbes.com/sites/briankoberlein/2016/04/26/looki...




>We call it "dark matter" because our formulas give the right prediction if we add a lot of mass where we see none, that is, if we assume there is some matter there that does not emit or interact with light.

I'm not a physics expert so forgive me if this is a naive take, but could black holes just sitting around, too far from other objects to have a visible effect, account for this unaccounted mass? IIRC they're small in size but incredibly heavy, and if space is infinite then they could just be sitting around taking up mass.


Someone has thought of this, mainly involving primordial black holes. IIRC the idea is that right after the big bang, when the subtle density differences in the gas permeating the entire universe caused pockets of gas to contract and eventually form stars, some pockets would collapse into black holes directly.

edit: I should add that they can't be regular black holes formed from stars, because then they'd have the same distribution of the stars in the galaxy they formed. While the the stars in a galaxy are most numerous near the center and get less numerous away from the center, the mass we have to insert in the form of dark matter to make the equations add up has to be evenly spread from the center and far beyond the visible edge of the galaxy.

The problem with this primordial black hole idea is that having lots of such black holes about would leave a very tell-tale signature thanks to gravitational lensing, which would be very strong near the event horizon. So far, no searches have found this.

Here's one news article describing the results of a recent search: https://physicsworld.com/a/supernovae-reveal-that-primordial...

See also https://en.wikipedia.org/wiki/Primordial_black_hole


It makes me feel good I came up with a similar theory to what a trained physicist did, even if it's likely wrong...

Thanks for the links, I'm on a wikipedia binge now :)


You're welcome!

I think part of the issue with dark matter and similar is that the main stream mostly hears about the theories that are the best candidates, and not about all the alternative explanations physicists have thought about but discarded due to some more or less obvious issues. Most won't even make it to arXiv, since they'll be eliminated by some rudimentary cross checks.

So the public might get the impression that physicists don't even try to come up with alternatives, but that couldn't be further from the truth.




Guidelines | FAQ | Lists | API | Security | Legal | Apply to YC | Contact

Search: