One theory of dark matter is that it's strange quark antimatter. An asymmetry in the behavior of quarks and antiquarks in the very early universe would have led to antiquarks being preferentially squirreled away in tiny ultradense nuggets of quark matter. While explaining dark matter, this would also explain the observed matter-antimatter asymmetry. Or, rather, it would explain it by saying matter and antimatter are present in equal amounts, but the antimatter is in a different form.

If this theory were true, tiny nuggets of this antimatter would be passing through the solar system all the time. Perhaps a future society could detect them and somehow trap some for use as an energy source.

> The current best guess is that there is some matter that we can't see for some reason. Nobody is sure what it is.

My pet conjecture (it's not detailed enough to be a hypothesis) is that this is related to the baryon asymmetry problem.

The antimatter symmetry problem is more than just baryons, despite the name, as we also have more electrons than positrons, not just more protons/neutrons than anti-protons/anti-neutrons.

There's a few possibilities:

1) the initial value just wasn't zero (an idea I heard from Sabine Hossenfelder)

2) the baryon number is violated in a process that requires conservation of charge

This would suggest antiprotons or antineutrons do something which involves the positron at the same time, so perhaps the anti-neutron is weirdly stable or something — neutron decay is a weak force process, and that can slightly violate the charge conjugation parity symmetry, so this isn't a completely arbitrary conjecture.

If we've got lots of (for example) surprise-stable anti-neutrons all over the place… it's probably not a perfect solution to the missing mass, but it's the right kind of magnitude to be something interesting to look at more closely.

3) the baryon number (proton/neutron/etc.) and/or lepton number (electron/positron/muon/etc.) is violated in a process that does not require conservation of charge.

If you have some combination of processes which don't each conserve charge, you're likely to get some net charge to the universe (unless the antiproton process just happens to occur at the same rate as the positron process); in quantum mechanics I understand such a thing is genuinely meaningless, while in GR this would contribute to the stress energy tensor in a way that looks kinda like dark energy.

But like I said, conjecture. I'm not skilled enough to turn this into a measurable hypothesis.

> If you want some antimmater, you can go to your nearby physics suply store and buy some radioactive material that produce positrons

Or buy some bananas. You'll get a positron every once in a while from the occasional Potassium-40 decay.

> Anyway, some people don't like "dark matter" and prefer to change the theories

It seems a bit more complicated than that, mostly because the vulgarization often available too is quite bad to explain the issue.

My understanding:

- Our current theories fail to predict/match an array of observations, as if more matter than what we can detect exist. Some scientists called that the "dark matter problem", that's what most physicists working on the subject refer to when they talk about "dark matter".

- Every theory you talked about: dark matter big objects, dark matter particles AND the "change the theories" (i guess you talk about the Modified Newtonian dynamics, where you alter Newton's second law at low speed to match some observations) are dark matter theories: theories that tries to explain why the universe act as it is, not matching our current theories, either by adding new things, or by modifying our discovered laws to match our observations. Each of those theories have multiple branch investigated.

- the "dark matter particle theory" is sometime vulgarized as "dark matter" on podcasts or in books/articles. This is because more scientists work on particle physics than on gravity or astrophysics (my country present like 3 astrophysics thesis each year, and dozens of particle physics thesis). I think this caused a huge misunderstanding.

- Some people with a common understanding (like mine, i meant non-physicists, it's absolutely not derogatory) like MOND because philosophically it is quite nice, and also tend to draw in people with minority/anti-etablishment habitus[0] (cf: most physicists working on those subjects are particle physicists). I'm not saying this theory is worse than the others at all, i'm just saying that the kind of layperson drawn to it can be _really_ sure they're right and profess their beliefs everywhere, and sometime claim that "MOND isn't dark matter", when they really confuse dark matter as a problem to be solved with "dark matter particle theory". Misunderstanding happen to everyone btw, it's really not a big issue.

In case you did not talk about MOND but about theories that claim that the issue are with our tools to observe at a distance, some theories include that to explain some of the inconsistencies, never all of them, and those theories seems to really be a minority atm, so hopefull it wasn't about that.

[0] Also, those habitus seems to draw in grifters who know they can make quick bucks by selling books/conferences if they look convincing enough, which is why MOND has a weird reputation now, but absolutely serious physicists and mathematicians work on the subject very, very seriously.