Is it just me or does this seem a bit like saying a landline is a wired cell phone? I thought dark matter represented what's missing, so if it's not there then nothing is missing, right?
We know that dark matter makes up a large fraction of the universe's overall mass. And we observe dark matter (indirectly) in just about every galaxy we look at. This makes sense, since we believe that galaxies are formed when local concentrations of matter collapse under their own gravity; you'd expect this process to affect both types of matter the same way. When we see a galaxy with no dark matter, it suggests that something unusual is going on, in a way that affects dark matter differently.
It's possible, but I think it's worthwhile to examine the intuitions that so many people have that leads them to dismiss the notion of dark matter, or at least the more "exotic" sounding theories like WIMPs.
We know that every particle we've been able to track down and verify the existence of responds to the gravitational force. We also know that some particles (like neutrinos) have virtually no interaction with anything other than gravitational force, to the point where the vast majority of them will pass through the entire earth undetected and having virtually no effect on anything or anyone.
The notion of particles that are even harder to detect than neutrinos doesn't seem that absurd. It's kind of anthropocentric to assume that most of the mass in the universe is going to be the same kind of mass that we are, or that we are at least capable of easily observing.
And think about what it would imply if there were no dark matter. It would almost imply that, as of the late 20th century, humanity finally had a basic comprehension of all of material existence.
The universe that humanity understood before Copernicus and Galileo is less than 1% of what we can see now. If our math is right and there really is enough dark matter and dark energy to throw off our models, then the universe we can see now is about 5% of everything that exists. That seems pretty plausible for a civilization that can barely get out of its own atmosphere.
Sure, it's just a lot more likely that it's stuff we can't see than our fundamental understanding of a loooot of other things is so incredibly wrong while still being very accurate in practice. I'm sure there are people putting effort into trying to reconcile things in that direction though.
I mean, the short answer is that, of course dark matter is a hack and our understanding of the universe could be wildly wrong or grossly incomplete.
The longer answer is that dark matter isn't particularly special or unique in this. We make observations, and where they don't match our expectations, we hypothesize new things to fill the gaps. Sometimes they're borne out by later observations, sometimes not.
We've hypothesized the existence of planets to explain deviations in planets' orbits. Sometimes the hypothesized planets existed, sometimes they didn't. In the early days of chemistry, we came up with a whole bunch of elements; sometimes they turned out to be compounds that were particularly difficult to break down. We came up a whole host of particles and subparticles, and the ones that we still think are real aren't necessarily the ones you would have guessed at the time they were proposed. If you pick up an old text book on quantum mechanics, you'll find they keep referring to it as "the new quantum mechanics", not just because it was being nailed down at the same time the text books were written but to distinguish it from "the old quantum mechanics" from twenty years earlier that it was replacing.
Sometimes the hacky models are the best we have to offer; most people will call the Standard Model of particle physics just a hacky description of the observed phenomenon, but all the elegant models to replace it haven't really paid off.
It's possible that dark matter will go the way of luminiferous aether, but so far all the observations astrophysicists are making really jive well with the idea of dark matter, so for now it gets to stay in the "weird things about the universe many people don't like" category instead of the "stupid ideas no one can believe that people used to believe" category. I mean, it makes me angry that a neutron isn't just an electron and proton mashed together, but them's the breaks.
Surely all theories are amendable given appropriate data. But we do have a few direct examples of “darkish matter”. Neutrinos definitely exist and pass through the sun faster than their counterparts.
Dark matter is only “missing” in the sense that an explanation of it is missing from established physical theories. It (whatever it is) certainly appears to be present, because it has a major effect on observed galaxy behavior. Otherwise there would be no need to bring up the topic!
I get that, it's not that I don't get the reasoning. It's just that it seems backwards to me (like in the phone example). But maybe my comment was an artifact of the fact that I (layman) haven't really bought into the whole idea that we really ever had any "theory" for dark matter in the first place. To me the appropriate headline would've been more like "astronomers find the first galaxy that actually behaves as theories predict".
I don’t think we do have a working theory that explains all the observed evidence. We know the theory without dark matter is wrong, because the vast majority of galaxies don’t agree with it. But we don’t have a new theory that holds together that does explain the observations, with or without dark matter.
Basically “dark matter” just means “something that makes the observations work” and the choice of words “dark matter” is because it would work to have some kind of yet-unexplained matter that doesn’t interact with the matter we know about. It looked like it might also work to change the theory of gravity, in which case that was a bad choice of placeholder name — but now this is a counterexample to that idea.
I don’t know the math, but my intuition based guess is it’s related to the range of interaction possible with other particles through QMs probability functions. Ie it’s important to remember that at quantum scale matter/energy transfer through space as probability density waves. Dark matter are at a low bar of the interaction distribution. Something about the empirical values of those equations gives way to many more particles at that low end than expected, but their existence couldn’t ever be ruled out.
Dark matter isn't "missing". It's invisible. It's something that has gravity but can't be observed. This galaxy is weird because it's mass is equal to the sum of its stars. In every other galaxy, its hundreds or thousands of times more heavy than all the stars put together.
Is it just me or does this seem a bit like saying a landline is a wired cell phone? I thought dark matter represented what's missing, so if it's not there then nothing is missing, right?