# Why do physicists assume that dark matter is weakly interacting?

IceCube, XENON, etc, keep yielding negative results. If dark matter exists, it doesn't interact with baryonic matter at the energy ranges they can detect. The response is to build even bigger detectors to search for even fainter energy signatures.

Why? Is there evidence that dark matter is supposed to have weak interactions (instead of gravity-only)? Or is it just searching for your keys under the lamp post (i.e. it's the only possibility that we have a way to detect)?

• It is worth noting that the 2-3 generation of WIMP detectors before the current one were never expected to actually turn up answers unless they got very lucky indeed. It is only recently that they've gotten up to a scale where they have a chance of ruling the idea out. So why were the previous generations built? As test-beds and technology demonstrators. Jul 31, 2018 at 1:04
• It is worth recognizing that direct dark matter detection is only one of many experimental approaches to the dark matter issue. There are also LHC searches for candidates, sterile neutrino searches, cosmic ray searches, & astronomy research in areas such as: lensing, N-body simulations, observations of galaxy scale structure and dynamics, 21cm measurements, studies of colliding clusters, gravity waves, etc. The study of dark matter phenomena is one of the only areas of fundamental physics informed by huge volumes of new data every week from many different kinds of independent sources. Jul 31, 2018 at 1:21
• LambdaCDM, the "Standard Model of Cosmology" (which has been a great success at the CMB level) assumes that dark matter is "almost collisionless", an assumption that poses its own difficulties when trying to reconcile the data at the galaxy scale with that model. Jul 31, 2018 at 1:26
• The lamp post joke works because the guy lost his keys elsewhere and knows it. If you don't know where you lost the keys, starting with lampposts is a good idea, since while having the same chance of having the keys, it has a lot higher chance of finding the keys. We don't know where the keys are, but we have a few ideas - starting with the ones easiest to check is pretty reasonable :) And ruling out weak interactions would be pretty helpful either way - particles that don't interact with our matter in any way other than gravity would be so cool :P Aug 1, 2018 at 6:57

The short answer is that they don't assume that.

But among all the proposals that remain for what dark matter might be, weakly interacting stuff is the easiest to detect,1 so that is what is getting the money right now.2

And that is not unusual. The history of missing-mass/dark-matter is one of proposals being made and then ruled out one-by-one, in order of ease of accessibility. WIMPs are just the latest candidate to get top-billing. MACHOs were hot when I was in college but were largely disposed of in the nineties and naughties. Before that, decades were spent with ever improving telescopes in wider and wider bands just ruling out many of the ways that ordinary matter could be hiding in plain sight (gas and dust, mostly).

And there are additional possible candidates in the theoretical catalog. I think that sterile neutrinos and/or axions will be next up if WIMPs are convincingly ruled out.

1 There is a caveat here in the form of sterile neutrinos which are not "detected" exactly but can be deduced by finding the three-flavor mixing matrix to be non-unitary. This is a hot topic again because MiniBooNe has recently announced an improved analysis of a larger data set in which the low-energy excess remains and the $\theta_{13}$ efforts have paid off in a big way so we've close to being able to quantify the unitarity (or lack thereof) of the matrix with some precision.

2 WIMPs in a particular mass range also offer the possibility of explaining additional features of the universe which makes them attractive for a second reason.

• In other words it is "just searching for your keys under the lamp post". Another point is that there is a huge lag between the decision to conceptualize, fund, build and the collect data from an experiment and the time it starts publishing results. Existing experiments were put in motion based upon science as of a decade or so ago, when electroweak scale supersymmetry, whose dark matter candidates are predominantly weakly interacting, looked much more promising than they did post-LHC results and prior to some key astronomy data. There was also a theoretical expectation that didn't pan out. Jul 31, 2018 at 1:09
• ^That. On the upside, work on these systems has meshed well with work on neutrinoless double beta-decay and together they have shepherded at least three distinct detector technologies through huge leaps in capabilities. The things they are doing in zero-background detectors today are astounding. Jul 31, 2018 at 1:13
• There is at least one axion detector ADMX in operation. It's sensitive enough to've already started culling the theoretical models of them by failing to detect them. The people running it apparently expect the current upgrade to be sufficiently sensitive to be the "definitive" version of the experiment. Assuming the sources for the WP article are correct, it looks like axions could be either found/ruled out around the same time as WIMPs. (Caveat, my knowledge of the subject stops at a few general interest level articles.) Jul 31, 2018 at 15:23
• There's another big point here. Back when people talked about the WIMP miracle, they thought that they really would be Weakly interacting (note the capital W, though more likely Z). We've long since ruled that out. BUT, if there is a particle out there with mass, it MUST couple to the Higgs. So you can ALWAYS write down a diagram with a Higgs propagator, which is still much much more strongly coupled than purely gravitation. Jul 31, 2018 at 18:46
• @LorenPechtel They are searching in the "relatively" easy place, and in lambdaCDM a particle with only gravitational interactions is a preferred fit, and W and Z boson and Higgs boson data have strongly disfavored any DM particle under 62.5 GeV that interacts via the weak force, so it is pretty unlikely. You have to go to unprecedented microweak charges, etc. to fit the parameter space. Jul 31, 2018 at 23:21

It's not just the "look under the lamp post" effect. There's also the "WIMP miracle". A new heavy (i.e. about the mass of the top quark, the heaviest SM elementary particle) weakly interacting particle would have an annihilation cross section of about $10^{-26} \text{ cm}^3/\text{s}$. Very general thermodynamic principles predict that thermal production of dark matter in the early universe could only lead to the observed density of dark matter if the dark matter has a similar cross section. This similarity suggests that dark matter might consist of heavy WIMPS.

Note: this answer is similar to this one also written by me. That question is very closely related and an interesting read.

Why do physicists assume that dark matter is weakly interacting?

They don't assume that.

To answer your question, you need to understand how dark matter was hypothesised, so here is a summary:

Using supercomputers, physicists were simulating the Big Bang and the formation of the Universe, applying Einstein's theories of special and general Relativity and Quantum Mechanics, experimenting with different variables to try to arrive at a system similar to our world as it is currently.

As they experimented, they found that in the simulations generated by the supercomputers, the matter formed attracted each other too weakly; matter and gas were flung out too far during the Big Bang and could not "clump" together to form stars or planets.

They tried adding some "dark matter"; matter which did not interact with the strong nuclear, weak nuclear, and electromagnetic force, i.e. it only interacted with ordinary matter gravitationally. This "placeholder mass" solved the problem, and the digital model successfully evolved to the system of the cosmos we observer today.

The intriguing thing was that ~$$85$$% (!) of the universe had to be made up of this hypothesised "dark matter" so that it formed correctly.

Conclusion: the universe can't have existed without this mass made up of WIMPs (Weakly Interacting Massive Particles). So let's go look for it!

Dark matter is called dark because it is hard to detect, even though it is greatly abundant. Physicist don't assume that it is weakly interacting, it was named "dark" because it is so.

• This answer does not explain why they should be weakly interacting rather than not interacting at all. Nov 17, 2020 at 17:29
• @RobJeffries it does—dark matter was hypothesized to provide the gravity required for the universe to form, otherwise physicists wouldn't have dreamed it up in the first place Nov 17, 2020 at 19:04
• Weakly interacting means interacting via the weak force. Dark matter is called dark because it doesn't interact electromagnetically. Nov 17, 2020 at 20:42