[xnoreq]

May I get your thoughts on this idea of mine: We all know the marbles on a rubber sheet experiment. Masses produce "dents" in spacetime that "pull" on other objects.

Couldn't it be that that these dents are balanced out by bulges between the masses, that would additionally "push" smaller masses (like solar systems) towards larger masses (like the centers of galaxies)?

In an experiment it would look like this: the rubber sheet seals the top of a container filled with water. Now if you push down at some point, the increasing water pressure will push up all around that point. If you now added a marble onto that bulge, it would roll down a steeper angle than it would in the normal experiment ... just like if there was extra mass ("dark matter").

[ninkendo]

Sabine Hoffensteder had apparently the same intuition as you and set out to develop a theory of dark matter as being anti-gravitating matter. It didn’t work out…

https://backreaction.blogspot.com/2021/11/does-anti-gravity-...

> Because I had this idea that anti-gravitating matter could surround normal galaxies and push in on them. Which would create an additional force that looks much like dark matter. Normally the excess force we observe is believed to be caused by more positive mass inside and around the galaxies. But aren’t those situations very similar? More positive mass inside, or negative mass outside pushing in? And if you remember, the important thing about dark energy is that it has negative pressure. Certainly if you have negative energy you can also get negative pressure somehow.

> So using anti-gravitating matter to explain dark matter and dark energy sounds good at first sight. But at second sight neither of those ideas work. The idea that galaxies would be surrounded by anti-gravitating matter doesn’t work because such an arrangement would be dramatically unstable. Remember the anti-gravitating stuff wants to clump just like normal matter. It wouldn’t enclose galaxies of normal matter, it would just form its own galaxies. So getting anti-gravity to explain dark matter doesn’t work even for galaxies, and that’s leaving aside all the other evidence for dark matter.

She goes into a lot more detail in the video (the blog post is just a transcript), but hearing her describe the thought process is interesting because she spent a lot of time on this theory (I’m assuming after she got her phd), so you’re in good company for coming to this initial conclusion at least :)

[xnoreq]

Thanks. I don't see what would clump, because it's not negative-mass matter floating between galaxies that I'm suggesting.

Just like entangled particles that are lightyears apart, when you measure the angular momentum of one, you know the other's because of conservation laws and not because of some hidden object nudging the particle the right way either.

Spacetime bulging the other way ("anti-gravity") would simply be a feature of spacetime to conserve some other property. In my example that would be the water pressure that wants to stay constant.

Some commenter below mention that this is like a waterbed. I like that analogy. If you put a large marble on a waterbed, not only will it create a trough, it will also ever so slightly lift everything around it.

[rrauenza]

Sounds like a waterbed… and there are google hits of other people with the same question! You are not alone.

https://science.slashdot.org/story/20/11/22/1714220/to-expla...

[Ygg2]

Another fun possiblity I remember from Universe in a nutshell, is that Dark matter is gravitation leaking from other branes.

In your analogy rubber sheet analogy, it's about stacking another rubber layer on top of current one.

It has no prediction powers , but it would explain things like why gravity is so weak and why we can't detect it.

[ggm]

Analogies are great for explaining versions of complex theories, but they rarely prove or disprove the theory because they aren't a 1:1 mapping to the theory, they're analogous. Similar. But similar is not "is the same"