On the 19th century astronomers noted that the orbit of the planet Mercury didn’t quite match what was predicted by Newton’s laws. To explain this they concluded that there must be another planet, which they named ‘Vulcan’ , inside Mercury’s orbit. They attempted to calculate the size, mass, and position of a body that would account for it.
The trouble was, they couldn’t actually find it. They also had to come up with explanations why it was invisible.
This lasted until Einstein worked out an improved model of gravity. Under Einstein’s model the orbit of Mercury made perfect sense without needing an invisible planet.
And these days I hear about a thing called “dark matter”. Apparently, the best models of the Big Bang don’t work unless we assume that there’s ten times more mass in the Universe than we can observe. So scientists have created a concept of “dark matter”, that accounts for 90% of mass in the universe, but we can’t see it.
To my (thoroughly inexpert) eyes it sounds like another Planet Vulcan. I don’t pretend to have any real understanding of the subject, but maybe the need for dark matter will be eliminated when someone comes up with a better model of the Big Bang?
So, is dark matter actually real? Or is it another imaginary planet?
Dark matter is also needed to explain the rotation of galaxies and the behavior of galaxy clusters. Basically, our observations of these things only make sense if there is a lot of extra matter that we can’t see, given our current understanding of gravity.
Some scientists have proposed that our current understanding of gravity is wrong at large scales, like galaxies. But no one has come up with a consistent alternative theory of gravity that fits our observations.
As I understand it (and I crave assistance), dark matter is well supported (better than Vulcan) because it explains both features of the Big Bang, and details of modern galactic dynamics, without ad hoc parameters to fit both types of observations. The non-dark matter theory proposed by some (MOND [MOdified Newtonian Dynamics]) does seem to me to be Vulcan-like - it is tuned to explain some galactic dynamical features, but doesn’t explain all of it, and has free parameters - and doesn’t incorporate relativity.
“Dark matter” is a placeholder for some amount of mass (or fields with mass-like effects) which we can’t directly observe and for which there is no explanation within the Standard Model of particle physics or fully explained within cosmology. It is the simplest solution to the ways in which we see large scale structures move such as the rotation of galaxies which should behave like a fluid with the arms constantly stretching out but instead seem to rotate at large scales like quasi-rigid discs, as well as gravitational lensing effects and the apparent mass that should exist to explain why galaxies are clumping together in cosmic-scale large structures instead of flying away from each other.
We have no idea what particles or fields could actually comprise “dark matter”, but the alternative hypotheses such as Modified Newtonian Dynamics (MOND) or variable ‘constants’ of physics, or so forth is that they require arbitrary tuning and a bunch of ad hoc rules to make them work, whereas just applying mass of a certain density and distribution neatly gives the observed dynamics without fudging things (at least, on the ‘local’ scale of galactic clusters). But whatever it is comprised of is unknown, and could be something far more bizarre than even the most radical hypotheses have proposed.
Honestly, if there weren’t any dark matter, that would be rather surprising. Why should all of the mass in the Universe happen to interact through the electromagnetic force? Indeed, we know of at least some particles that don’t (though there are other lines of evidence that those particles don’t make up more than a small fraction of the dark matter). And there are many different lines of observation that lead to the conclusion of dark matter, and all of those observations more or less agree.
Now, dark energy, that’s the really weird stuff. If it’s “stuff” at all. Fifty years ago, it was vogue to refer to phenomena of that sort as the “cosmological constant”, an at-the-time unobserved possible aspect of Einstein’s equations for gravity. Now, it’s more vogue to refer to it as a sort of substance which has very unusual properties. There’s not really any way to distinguish the two models, and so cosmologists basically just choose their description based on what seems more elegant to them.
There is a concept from particle physics, vacuum energy, that qualitatively has the right properties to account for dark energy, and I think that’s the reason why the “dark energy” concept is currently favored, but quantitatively, while we can’t calculate the exact strength the vacuum energy should have, we can estimate it, and the estimates are about 120 orders of magnitude off. Note: That’s not a factor of 120; that’s a factor of a hundred quintillion googol. Personally, I think that an explanation that’s that far off is worse than no explanation at all, but obviously others disagree.
You might be interested in the “Observational evidence” section of the Wikipedia article on dark matter. I’d be happy to help interpret any of those many pieces of evidence, as they aren’t always described in an accessible way. Also, that list is not complete.
Dark matter is one of the most solid “known unknowns” in physics today. It’s there.
I think it may be also comparable to the luminiferous æther. According to our current understanding of physics we need this for “it” to work. But maybe we need new physics and not dark matter.
Dark matter is the new physics. The wiki link above outlines the overwhelming and, in many cases, independent pieces of evidence for its existence. Dark matter is not a wild guess but a clean inference from all that evidence – an exemplar of Occam’s razor.
That analogy doesn’t really work for a couple of reasons. One is that the luminiferous aether was a legacy theory from a time before there was a consolidated theory of electrodynamics. It was based upon the idea that light was a wave conveyed through some medium rather than a self-propagating pulse of energy as described in Maxwell’s equations, but the medium was never adequately described in any material context, and in fact the properties were inconsistent and possibly self-contradictory. The other is that experiments to measure the directional variations of the luminiferous aether on the speed of light were completely inconsistent with theory; that is, that experiments (namely the Michelson–Morley experiments) produced observations completely inconsistent with the entire notion of a static background medium as the speed of light was found to be the same in all directions. The ‘theory’, such as it was, stemmed from a long-held assumption that had never been theoretically validated or experimentally or observationally verified, and was just a consequence of a totally wrong view of how light travels through vacuum.
‘Missing’ dark matter, on the other hand, is entirely consistent with observation (at least on scales where we can make such measurements), and we just lack the actual particle or field that would produce the required mass. Since it is widely hypothesized to not interact on the electromagnetic spectrum, we don’t have any way of making ‘direct’ observations beyond the gravitational effects it has at galactic-sized and larger scales. It is definitely an extension of current physics because there is nothing in the Standard Model that even suggests what that particle could be but frankly any other possible explanation would also be ‘new physics’. It is, however, consistent with General Relativity as it doesn’t introduce any new force or modification that has to be arbitrarily tuned to produce observed results.
This might be moving too far into opinion rather than FQ, but for the physicists here, do you think “what is dark matter” is a solvable question? With enough time, thought, experiments, and scientific advancement, could we somehow figure out (and maybe observe) the particle or field responsible?
To the extent that any unknown phenomenon on the frontiers of physics may be unsolvable until a solution is found, with (missing) dark matter we at least know the parameters of what to look for, and as importantly, what it cannot be. Discovery by process of elimination isn’t as sexy as having a proverbial “Eureka!” moment but it is actually how science progresses most of the time.
A very recent article on the search for one possible (and widely considered most likely) candidate for dark matter:
Yes, and this is an active area of research. There was a thread from (gosh) ten years ago that touches on some of the aspects of terrestrial detection. Happy to revisit any of those points. The practical side of making measurements like these is often not given the same emphasis in pop sci as is highfalutin theoretical speculation, but the stories are just as compelling (or, often, arguably more so).
To my mind, the most compelling evidence of dark matter is gravitational lensing. MOND cannot explain it and the baryonic mass of galaxies is insufficient. All the matter, both dark and baryonic, is needed. Here, for example, is a newly discovered Einstein Cross that has all five images (the center image on most crosses is obscured by a foreground galaxy):
That isn’t strictly true, but correlating a MOND model to observed gravitational lensing requires tuning parameters to within a specific range. For (missing) dark matter, you just have to assume a certain distribution of mass with a self-consistent gravitational field, which is also a kind of ‘tuning’ but doesn’t require wild gyrations of reasoning to explain why GR is otherwise a well-tested theory but doesn’t predict observed dynamics of galaxies and galaxy clusters.