About the dark matter thing, I have heard about a few possible candidates that are being examined more closely at the moment. One are the neutralinos, the other are black holes between 20 and 100 solar masses, which are the only size possible for dark matter and which have been discovered more often with LIGO and the other detectors for gravitational waves. What do you think about those theories? (And about MOND, that hardly ever gets coverage for the general public)
Thanks for asking! To start out, we don’t know much about dark matter, but here’s what we do know: Stars are orbiting galaxies too fast for the visible matter to be the only thing holding the galaxy together; in order for the speeds of stars orbiting to be reconciled with the amount of mass we can see, we’ve concluded that there has to be about 5-10x as much matter that can’t be seen because it doesn’t interact with light, giving it the nickname “dark matter.” Based on rotational curves (which show the speed of stars rotating around the galactic center at a given distance from the core), we know that the dark matter extends in a “halo” significantly larger than the visible galaxy. (image source)
This is a rotational curve; as you can see, the rotational curves don’t follow the path expected at all. Astrophysicists did a bunch of math and figured out that the dark matter must be in a huge halo around the galaxy: (image source)
Now for the different theories you talked about! I’m not too familiar with neutralinos; from what I (just) read, they’re theoretical particles with a lot of mass, and one of the predicted ones are WIMPs (Weakly Interacting Massive Particles - don’t you love how astronomers name things), which is a good candidate for dark matter, but hasn’t yet been proven. A lot of articles I’ve read about dark matter searches spent some time talking about WIMPs so they’re probably a good candidate and could end up being right. Another particle had the acronym MACHOs and i believe it also has to do with dark matter.
Black holes are definitely not the source of dark matter, because there is 5-10x more dark matter than luminous matter in the universe, meaning that there would have to be at least 109 or more of these black holes (essentially one black hole for every 1-10 stars), so we can write this theory off. Also, this theory wouldn’t match the observed rotational curves - whatever dark matter there is, it has to be whizzing around in the dark matter halo.
MOND, or modified-newtonian dynamics, is almost definitely not true. The theory of gravity (general relativity) is pretty pretty solid and extremely unlikely to be wrong, so redoing our entire theory of gravity to fit dark matter is probably not the answer; this is why it gets essentially no coverage.
If you want my personal opinion, I think the most likely candidate would be a massive but very weakly (if at all) interacting subatomic particle that we haven’t discovered yet. From what I understand, we’re trying to figure out how to detect them, but it’s very very hard since these particles don’t interact with light (and only really interact gravitationally).
If you want more information about dark matter, you should check out this post by @quantanaut about what we know (or more accurately don’t know), along with the dark matter tag on my blog. ps im sorry I literally typed you an essay but I wanted to be thorough. If i did a poor job explaining anything or you want to talk more about it or want clarification or something, feel free to shoot me another question!!