Understood that Dark Matter is somewhat conjectural and there are multiple hypotheses about its nature, but maybe someone can enlighten me based on what we think we might know, or what we suspect about the nature of Dark Matter…
Is Dark matter going to be one kind of thing - that is, a distributed collection of some singular particles or other entities that are all the same?
Or is it going to be capable of complex organisation similar to ordinary matter - is there going to be a Periodic Table of Dark Matter, or similar?
It could be a single type of particle. It could be a handful of types of particles. It could be many handfuls of types of particles. At the moment, one can mostly get away with just a single massive, weakly interacting particle. If you introduce additional (fundamental) particles you have to introduce reasons why the heaviest ones haven’t decayed into the lightest one over the past 13 billion years, but that’s doable. (It just constrains the sorts of models you work with).
It is certainly possible, though, that dark matter particles interact via some as-yet-unknown force, and this is a hot topic right now. By its very nature, we would have no easy time directly accessing/testing this force. However, if bound states of any durability could form, they would affect the development of structure (galaxy clusters, etc.) in the early universe, and we can look for signals there. In just the right kind of bound-state model, the excitated state spectrum could even be used to explain some of the inconsistencies seen in the present collection of direct, indirect, and cosmological dark matter detection data.
To be sure, though: we currently just don’t know.
(I’ve personally become a fan of a richer dark sector lately, and I’m glad it’s being pursued. Once you cross a certain threshold of complexity (such as introducing a new “dark force”), it starts to seem more Occam-respecting to let the dark sector be as complicated as the “light” sector. That is, if quarks and leptons can have multiple forces and three particle families, why must the dark sector be limited to a single particle and/or a single gauge force?)
Has it even been ruled out that dark matter could be the same regular type of matter we see every day, just cold so it doesn’t radiate? Or if a lot of dark matter is inside black holes, it doesn’t matter what sort of matter it started out as.
At least some of the dark matter is exactly that (heck, we’re made of dark matter), but what people usually mean when they speak of dark matter is the non-baryonic dark matter (i.e., matter whose mass comes mostly from things other than protons and neutrons). We can put strict limits on the amount of baryonic matter (or formerly-baryonic matter, like black holes formed from stars) via big-bang nucleosynthesis, and it’s not nearly enough to account for all of the dark matter.
Basically, the idea is that some isotopes (hydrogen, deuterium, helium-3, and assorted isotopes of lithium) were formed almost exclusively in the Big Bang, and the relative abundances of those isotopes depended on how many baryons were around to form them (if there had been more baryons around, then the bigger isotopes like lithium and helium-3 would be relatively more common). Based on the abundances of those isotopes we do actually have, we can calculate how many baryons were available, and extend that forward to how much baryonic matter there should be now.
Back to the original question, nobody knows, but my guess is that there are several different types, which are stable for different reasons. It doesn’t seem plausible to me that most of the matter in the Universe is all the same stuff. Again, I hasten to say that this is just a guess.
On the other hand, the private sector seems to be limited to a single gauge force: money, although it is carried by many different, interconvertible bosons.
But suppose a lot of non-baryonic matter ended up inside black holes. Black holes formed from WIMPS would be indistinguishable from black holes formed from baryonic matter. So we would see the influence of all this mass, but when we look for it, we find today that it’s all inside black holes, like the center of our galaxy.
There are observational limits to the ‘clumpiness’ of dark matter, and since black holes would be pretty clumpy, they aren’t very good candidates for dark matter.
Its also pretty hard to get non-baryonic dark matter into black holes, since the lack of electro-magnetic interaction means it won’t bleed off angular momenta and fall in like regular matter does.
Could dark matter form itself into stars and planets? It seems dark matter mostly hangs out around regular-matter galaxies like a big amorphous gravitational haze. Why doesn’t it form itself into gravitational clumps like stars and planets do, or get clumped together with normal stars and planets?
Short answer, it doesn’t clump significantly (at least, not on scales smaller than galaxies) because it doesn’t interact electromagnetically. That means a lot more than just “doesn’t emit light”: Most of the interactions between matter we’re familiar with are ultimately electromagnetic. Put two chunks of “ordinary” matter on a course moving towards each other, and when they get close enough, they might end up sticking together, forming a bigger clump. Do the same with two clumps of dark matter, and they’ll most likely just pass by each other and moving apart again, without any significant interaction.
Ah, I get it. Thanks. So why does it bother to clump around galaxies? Wouldn’t the movements of galaxies eventually stir dark matter into a more or less uniform mass throughout the universe?
If the only way Dark matter interacts with normal matter/energy is through gravity, if there was any significant amount in the solar system, shouldn’t we be able to detect it, by it’s effect on the planets/sun?
There is/would be plenty of dark matter in the solar system. But on such a small scale, compared to the galactic clusters on which dark matter really exerts its influence, the dark matter is so uniform that the solar and planetary motions are affected only negligibly, if at all. It would be like looking for the existence of mountains by analyzing a square centimeter of Kansas.
The thing about “dark matter” (if we’re not just talking about cold regular matter) is that it is weakly interacting except for gravity. That means that light would pass through it, so it would be translucent. The electromagnetic fields of regular matter wouldn’t interact with it, so it would pass right through a bucket, and through the floor, and through the Earth. You’d have a clump of it orbiting inside the Earth as if the solid rock was a vacuum. That depends on exactly how weakly the dark matter interacts with regular matter of course. And dark matter would weakly interact with itself, so a lump of it wouldn’t stay a lump. It wouldn’t expand like a puff of air, because gas molecules bump against each other and therefore spread out. Rather, each particle would just continue it’s newtonian path through the universe. And that means it would be really hard to gather up a bucketful in the first place, because the only way to interact with it would be by gravity. So they could be orbiting around a star or a planet or a galaxy or a black hole, but you couldn’t collect them without some science fictional method of gravity control.
Right. So imagine a bunch of WIMPs orbiting the Sun. They don’t clump up into bodies like baryonic matter, each particle just orbits away. And since each particle is individually very small, and it takes godzillions of protons and neutrons to produce a gravity field as weak as the Earth’s, you’d have to have godzillions of those particles concentrated into one spot to notice them. And since they don’t stick together, you can’t get such a concentration.
An analogy: Let’s say you have a glass of icewater. It’s mostly melted, but there’s still a little ice in the glass. Now, you spill it on the floor: Water and ice scattered all over the place. The liquid water is more abundant overall, but it’s spread evenly. Whenever you have a chunk of ice, it’s much more significant than the liquid water, right at that spot, but over the entire floor, there’s more liquid.
Now replace the water with dark matter, and the ice with stars or other clumps of normal matter.
Is there any currently conceivable way we’d find that our mass calculations are just wrong? Newton could have assumed there was some unseen force causing light to bend twice as much as he predicted.