Additional Notes

https://www.reddit.com/r/space/comments/6488wb/i_dont_want_to_be_anti_science_but_i_am_doubtful/dg05wx4/

Senno_Ecto_Gammat
757 points·1 year ago·edited 1 year ago
Below is basically a historical approach to why we believe in dark matter. I will also cite this paper for the serious student who wants to read more, or who wants to check my claims agains the literature.
1. In the early 1930s, a Dutch scientist named Jan Oort originally found that there are objects in galaxies that are moving faster than the escape velocity of the same galaxies (given the observed mass) and concluded there must be unobservable mass holding these objects in and published his theory in 1932.
2. Zwicky, also in the 1930s, found that galaxies have much more kinetic energy than could be explained by the observed mass and concluded there must be some unobserved mass he called dark matter. (Zwicky then coined the term "dark matter")
3. Vera Rubin then decided to study what are known as the 'rotation curves' of galaxies and found this plot. As you can see, the velocity away from the center is very different from what is predicted from the observed matter. She concluded that something like Zwickey's proposed dark matter was needed to explain this.
4. In 1979, D. Walsh et al. were among the first to detect gravitational lensing proposed by relativity. One problem: the amount light that is lensed is much greater than would be expected from the known observable matter. However, if you add the exact amount of dark matter that fixes the rotation curves above, you get the exact amount of expected gravitational lensing.
By this time people were taking dark matter seriously since there were independent ways of verifying the needed mass.
MACHOs were proposed as solutions (which are basically normal stars that are just to faint to see from earth) but recent surveys have ruled this out because as our sensitivity for these objects increase, we don't see any "missing" stars that could explain the issue.
6. The ratio of deuterium to hydrogen in a material is known to be proportional to the density. The observed ratio in the universe was discovered to be inconsistent with only observed matter... but it was exactly what was predicted if you add the same dark mater to galaxies as the groups did above.
7. The cosmic microwave background's power spectrum is very sensitive to how much matter is in the universe. As this plot shows here, only if the observable matter is ~4% of the total energy budget can the data be explained.
8. This image may be hard to understand but it turns out that we can quantify the "shape" of how galaxies cluster with and without dark matter. The "splotchiness" of the clustering from these SDSS pictures match the dark matter prediction only.
One of the recent most convincing things was the bullet cluster as described here. We saw two galaxies collide where the "observed" matter actually underwent a collision but the gravitational lensing kept moving un-impeded which matches the belief that the majority of mass in a galaxy is collisionless dark matter that felt no colliding interaction and passed right on through bringing the bulk of the gravitational lensing with it.
Evidence 10: Galaxies experience tidal forces that basic physics says should rip them apart and yet they remain stable. And the amount of unseen matter necessary to keep them stable is exactly what is needed for everything else.
11. There are counter-theories, but as Sean Carroll does nicely here is to show how badly the counter theories work. They don't fit all the data. They are way more messy and complicated. They continue to be falsified by new experiments. Etc...
To the contrary, Zwicky's proposed dark matter model from back in the 1930s continues to both explain and predict everything we observe flawlessly across multiple generations of scientists testing it independently. Hence dark matter is widely believed.
Evidence 11: Dark matter theories have been around for more than 80 years, and not one alternative has ever been able to explain even most of the above. Except the original theory that has predicted it all.
Conclusion: Look, I know people love to express skepticism for dark matter for a whole host of reasons but at the end of the day, the vanilla theories of dark matter have passed literally dozens of tests without fail over many many decades now. Very independent tests across different research groups and generations. So personally I think that we have officially entered a realm where it's important for everyone to be skeptical of the claim that dark matter isn't real. Or the claim that scientists don't know what they are doing.
Also be skeptical when the inevitable media article comes out month after month saying someone has "debunked" dark matter because their theory explains some rotation curve from the 1930s. Skeptical because rotation curves are one of at least a dozen independent tests, not to mention 80 years of solid predictivity.
So there you go. These are some basic reasons to take dark matter seriously.