The Reason We Haven’t Directly Detected Dark Matter

I highly recommend reading the book: we know no idea.

But dark matter no longer is particularly simple. At the time of its initial proposal it was little more than unobserved masses, which could have been something as simple as e.g. red dwarves. And so it made sense to reference dark matter as a place holder. But over time as we've discovered things are even more off, dark matter itself has started to morph into our 2000 pound Gaelic speaking deer. Now it comprises the vast majority of all matter in the world, it can't be baryonic, it mustn't interact with anything except itself and gravity, we need to have entire galaxies made of nothing but dark matter, and so forth, and so on. Add now add in the whole slew of experimental failures to detect it, as null hypotheses, and it features ever more exotic conditions as well.

And on the topic of those test failures, every single test we make to detect dark matter, which everything we hypothesize about it indicates we should be able to measure, says it's not there. This should not be a problem except for the fact that we've performed alot of experiments at this point to try to detect dark matter of all sorts, with increasingly high precision. And they've all returned a strong negative. With Bayesian logic it's almost surprising we haven't had a false positive yet, given all the failures.

I really do think the geocentric model has a lot in common here. Prior to Newton's discovery of strong mathematical indicators for gravitational mechanics, heliocentricism had long been proposed. And it was in many ways a much more simple explanation for many phenomena than geocentricism, which over the centuries became ever more convoluted and complex. The 'petal weaving' geocentric astronomic models are enough to boggle the mind even today. But one of the big problems was not even the church, it that there was an immense amount of academic work based around a geocentric model. As but one example astrology, which was considered a scholarly and academic pursuit in the past, was based heavily on the assumptions of a geocentric universe. For instance Mercury starting to 'go backwards' (which was required to glue together a geocentric astronomic view) is still a part of astrological 'theory' today.

Alternatives to dark matter look increasingly more likely with every failure of experiment designed to test it. And these alternatives need substantial refinement, but I think really the biggest issue with them is that if dark matter is not correct it would basically destroy more than a century of astronomical and cosmological work. Nearly everything we know about the cosmos is predicated on the assumption of dark matter. If it turns out dark matter is not the issue, we're looking at setting astronomical and cosmological work in many areas back a century. It would be, by fair, the biggest failure in modern science. So there is reason that alternatives to dark matter are held to a higher standard than dark matter itself, beyond simplicity - which dark matter no longer really is.

Hawking's "A Brief History of Time" is really good, and only has one equation. My mom read it, even.

> Because my intuition would be, if you loose normal matter into a gravity well, it will clump, even if it doesn't interact with the source of the gravity. Am I inferring correctly?

Now I have a little of Newton for you: look at our Solar system: you see the planets, and even more interesting, all the small asteroids circling around the Sun? Can you answer why don't they all fall to the Sun, but move in the orbits?

The way the gravitation works was not "intuitive" before Newton, 300 years ago, and now it's obviously still so for many non-professional readers.

What's actually happening, according to the dark matter model, and the dark matter actually more easily fits much more of our cosmological observations than anything else, is that there is a lot of dark matter but it is simply much more "spread" around the volume of the galaxies. And just like all the visible stuff of the whole galaxy doesn't fall to the galaxy center (like the planets don't fall to the Sun!), the dark matter remains "around" the galaxies, where more of dark matter is "outside" (as in "in the outer regions of it") than in the "inside" of the galaxy (and in the case of the "Bullet Cluster", that I've mentioned in some other comment, dark matter is obviously lagging all the movement of non-dark matter! (1)). That dark matter that is in the inside of the galaxies actually initially "clumped" somewhat, but that "somewhat" is, according to our estimates, significantly below what we are able to measure, when we're interested in the gravitational effect on the Solar system.

1) https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:Bul... and https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:1e0...

> What do you think of the possibility of 'dark sector' interactions?

It remains a possibility. It does not seem to be required by observation, though. Worse, if you move away from parsimonious non-interacting quantum field theories to more complex models, you have to suppress a lot of symmetries that inevitably produce observables which are not seen. Most people working with general relativity just use non-interacting scalar fields, but specific ideas about dark matter have to consider the ins and outs of gauge theory (e.g. does DM only feel gravitons and Higgs or does it also feel one or more of the other non-photon gauge bosons? if it feels the weak force, what goes on at electroweak scales? and so on...). The microscopic details of the microscopic alternatives within the broad family of QFT dark matter get hairy quickly, and there's very little astrophysical evidence to prefer one over the other (people favour axions or sterile neutrinos for reasons from within particle physics, and are looking for such things to complete their extensions to the standard model, they have to be very weakly interacting for particle-physics-in-laboratories reasons, but oh by the way as a side effect dark matter could be wholly or at least partially these proposed standard-model-problem-slaying particles).

> [what if we propose dark photons, dark atoms, etc.?]

One problem you run into is that if you can form composite dark particles analogous to atoms, or dark molecules, what prevents them from forming larger structures that collapse gravitationally? Likewise, if you can emit dark photons, you're removing momentum-energy from a particle in an orbit, and you would then expect the particle to fall into a closer orbit. Again, how do you prevent gravitational collapse? You might fix that by feeding back (squash DM together in galactic cores, release enormous "dark shine" dark-photon-analogues which then kick the massive DM particles into wider orbits, but it's like balancing a pencil on its tip; this is called DMAF (dark matter annihilation feedback), and is speculative. On the other hand baryon-flow feedback is a thing in solving e.g. the core/cusp density problem in particle dark matter, and that's a lot less speculative, because we know things like galactic jets are practically mandatory.

You're generally stuck with appealing to rareness, which is in conflict with Copernican principles which work really remarkably well in cosmology (and astrophysics too), or slowing down dark chemistry so much that it basically doesn't have to enter into equations anyway.

Carroll blogged about this a decade (!) ago (how to feel old: remember reading his cosmic variance blogpostings and making the discovery, pardon the pun, of how many years it's been since he stopped blogging there...) here : http://www.preposterousuniverse.com/blog/2008/10/29/dark-pho...

In astrophysics instead of using base-ten for enumerating interesting things in the sky, the counting system goes roughly: forbidden-everywhere, unique, mandatory-everywhere. If you introduce dark matter stars, you would expect there to be so many of them that you could not miss the Einstein lenses they generate. (Similar to MACHO hunting). Dark matter galaxies, being much more massive, would be even harder to miss. You will struggle to find a deep-field image that isn't filled with background galaxies (or clusters) lensing even more background ones. If there are dark galaxies, surely they would be in the foreground of some of the visible galaxies -- otherwise what prevents that?

Finally, we do have some gravitational structuring of dark matter; the standard description of structure formation requires it, and it's hard to get the late-time structures we see without dark matter filaments.

There's undoubtedly some meat on the Structure Formation wikipedia page and the things it links to, but a briefer and simpler overview is in the last three paragraphs at http://sci.esa.int/planck/51560-the-history-of-structure-for...