These articles always miss the mark because they get the history backwards. Modified gravity theories were taken much more seriously in the 1970s and earlier, where the evidence for dark matter was shakier. Cosmological and astrophysical observations from the 1980s to 2000s have vastly strengthened the case for dark matter, which is why it's the leading hypothesis now.<p>Popular articles won't tell you this, because <i>without exception</i> they laser focus on galaxy rotation curves, a piece of evidence that's nearly a century old and by far the weakest one. But by neglecting to mention the actual evidence we base our conclusions on, they (purposely or not) make us look like fools.<p>Furthermore, dark matter hasn't been ruled out. One particular candidate of what it could be (a WIMP) has been studied and mostly ruled out over the past 20 years, but by the nature of the business there are many possible candidates. And of course we've shifted attention towards those other options, which is exactly how science is supposed to work!
I used to work on dark matter experiments. (This was a few years ago now, and wasn't for very long -- I was helping out colleagues -- but I don't think the situation has changed much.)<p>I don't think anyone worth listening to considers dark matter to be a Universal Truth or anything. It's a crappy theory that needs better experimental evidence on the micro scale. It's just that all attempts to find that evidence have turned up nothing. (Except DAMA/LIBRA, the bane of the previously mentioned colleague's days and motivating factor for the experiment in question... but nobody actually believes them, so let's value their reports at "nothing".)<p>The biggest problem is that all alternatives to date are worse. They <i>all</i> <i>have</i> <i>problems</i>, problems of equal or greater severity than dark matter. That, more than any other reason, is the reason dark matter is still the leading theory.<p>I do strongly believe we should work on the advancement of non-dark matter theories. That's how you lap the current leader, after all. And neglecting alternative theories was what got us into a couple decades of string theory stagnation, a mistake we'd be wise not to repeat. If a new theory or new evidence for an existing theory emerges, dark matter will lose mindshare amazingly quickly.<p>There is, of course, a "nightmare scenario". The WIMP carries a weak charge, by definition. It was dreamed up that way to make it detectable and have a presence in the universe. (And there may have been a theoretical preference; I wouldn't know, I've always been an experimentalist.) But there's no reason it <i>must</i> carry a charge, weak, strong, or electromagnetic. That would make it really quite invisible to experiments. This may be the universe we live in.
I just find this odd... we've seen Dark Matter fill in the gaps of standard astronomy (general rel) models of galactic evolution, but it's been predicted and seen in multiple places over the last 50 years and the measurements keep getting stronger (cosmic background spatial distribution, galactic rotation, bullet cluster). Each measurement rules out some model explanation, and we search for different particles (WIMPs, MACHOs, neutrinos) that fit the models that work.<p>We still have interesting testable explanations... but let's give up now, it's boring? MOND again?<p>Sorry, axions are interesting and may explain other things (antimatter ratio etc) and if that fails I think others will pop up, that don't require reexplaining astronomy.<p><a href="https://phys.org/news/2020-06-case-axion-dark-gains-traction.html" rel="nofollow">https://phys.org/news/2020-06-case-axion-dark-gains-traction...</a>
> But now it’s time to consider...<p>Almost invariably when an article title says "it is now time to..." there has been no actual threshold that has been crossed making "now" special at all.<p>The growing types of evidence for a "dark matter" effect on large scales, vs. the absence of any small scale evidence for it, has consistently begged for new particle, gravitational, and other types of theory to be explored.<p>It is a strange situation to have something appear so consistent with one of our pillars (general relativity), while completely absent from the other (quantum mechanics).<p>I am crossing my fingers that any successful understanding of dark matter on the small scale might shed light on how those two pillars could be combined into a single theory. But any explanation will be most interesting!
If you enjoy science fiction, one of the better recent SCPs was about dark matter:<p><a href="http://www.scp-wiki.net/scp-4170" rel="nofollow">http://www.scp-wiki.net/scp-4170</a><p>SCP is a world-building collaborative creating-writing project, focused on sci-fi cosmic horror. Each entry is standalone, though there is also some canon which contributors are encouraged to follow.<p>SCP-4170 was generally received as a positive entry, though I read it as being far more sinister.<p><i>Miriam: [Request to still be taken seriously]</i>
MOND was a healthy idea to pursue, not because it was the correct alternative (I really have my doubts that it was) but because it was <i>AN</i> alternative.<p>Maybe some postgrad might have had less of the mainstream science groupthink indoctrination because of it.<p>That being said, dark matter is a real phenomenon: it is a measurable quantity of how wrong we are. Dark matter isn't the problem, WIMP is.
Doesn't the bullet cluster provide strong evidence for the existence of dark matter? Shouldn't all alternatives need to explain the phenomenon seen there?
> [Milgrom's alternative theory:] <i>But far in the outlying areas of the Milky Way, stars would feel a smaller gravitational force than previously thought</i><p>Isn't it the opposite?<p>Dark matter is extra matter which implies extra gravitational force to hold the galaxies together. If you want an alternative to dark matter, shouldn't the alternative theory predict a <i>bigger</i> gravitational force than previously thought?
I would enjoy an explanation as to why the distribution of normal and dark matter in galaxies is roughly the same. My ignorant intuition tells me that since normal matter is subject to many more and different forces than dark matter, one of them should be much more densely-orbiting the galactic core than the other. But they aren't.
I'm wondering always about the name "dark matter". It makes a good topic for all kinds of pseudo-science and scifi. The name implies something mystical and invites everyone to discuss a topic that no-one understands.<p>If the name was something else like "Rubin's matter" or "Unknown matter", maybe it wouldn't invite so much unscientific discussion and speculation.<p>I don't feel that everyone has an opinion about higgs boson, but you can start a discussion about dark matter with anyone. I'm not any kind of expert about the topic either, but could certainly start speculating about it with other laymen.
> But far in the outlying areas of the Milky Way, stars would feel a smaller gravitational force than previously thought from the bulk of matter in the galaxy;<p>Shouldn’t it be: "a stronger gravitational force than previously thought"?
A simple back-of-the-envelope estimate shows that it takes an -extremely- small amount of very low-density 'invisible' matter - assuming that it's spread out over the huge 3D 'spherical' volume of a galaxy - to make up the missing mass. On the order of a few -atoms- per cc.<p>Given all of the unknowns about interstellar/ intergalactic matter, including the density of EMF, it's not too hard to understand why the account sheet won't balance. It's a hard and wonderful problem.
What if dark matter is really just regular matter that is not visible through our instruments?<p>Like large swaths of ice or dust in the voids, similar to our kuiper belt that is difficult to analyze.
What, exactly, is the prediction of dark matter, compared to MOND (or related theories)?<p>As a layman it seems like:<p>1. Dark matter is not universal (i.e. it is a "random" property of space)
2. Dark matter interacts with itself (and other matter) via gravity.<p>Is that all there is? I mean in such a case it seems to be no wonder that it fits the observations better, as DM theory is by far less constrained then MOND.
While the stationary background universe model proposed by Peter Ostermann doesn't rule out „dark matter”, it at least doesn't require „dark energy“ in order to match with observational data.<p><a href="http://peter-ostermann.org/assets/sum_os14.pdf" rel="nofollow">http://peter-ostermann.org/assets/sum_os14.pdf</a>
MOND is appealing from a purely philosophical point of view: we have a separate (“modified”) theory for when thing are very small, another one for when things move very fast, so, the need for another modification for when things are very large should not be seen as entirely unexpected, should it?
Does anyone know if there are any effects from inflation [1] at galactic boundaries?<p>[1] <a href="https://en.m.wikipedia.org/wiki/Expansion_of_the_universe" rel="nofollow">https://en.m.wikipedia.org/wiki/Expansion_of_the_universe</a>
I've often wondered if there is an equivalent of the magnetic field for gravity. The magnetic field seems to be a consequence of resolving charge interactions for relativistic observers, and the same could be true for gravity.
Can anyone Please point to the model used for the "expected" galactic rotation curve? This whole dark matter thing started with observation not matching prediction. I'd like to scrutinize the prediction.
Yes! I've been saying for years that dark matter is just a mathematical convenience and that we don't have any real evidence for its existence. Our model w repsect to it needs rethinking
Maybe physics needs a more holistic light theory based on stillness and the two-way motion of each phenomena in the cosmos: <a href="https://wikischool.org/divided_light" rel="nofollow">https://wikischool.org/divided_light</a>