What Do Dark Matter Theorists Think The Big Questions Are?

A Snowmass 2021 paper on "Astrophysical and Cosmological Probes of Dark Matter" sets forth in its introduction what it sees the big open questions are from a dark matter particle perspective( form the introduction, bold emphasis mine, italics in headings from the source, with my commentary outside block quotes after each major question):

• Is the Cold Dark Matter paradigm correct?

In the CDM paradigm, dark matter is collision-less and non-relativistic during structure formation. A natural consequence of this is the prediction of an abundance of low-mass dark matter halos down to ∼ 10^−6 M. 

Observations that provide information on the matter power spectrum at small scales and various redshifts, therefore, will play a pivotal role in confirming the CDM hypothesis. 

Evidence of small-scale power suppression could, for example, suggest that dark matter is warmer (i.e., not nonrelativistic) during structure formation, is not collision-less, is wavelike rather than particle-like, or underwent non-trivial phase transitions in the early Universe. 

As we will discuss, upcoming astrophysical surveys have the potential to start probing halo masses to much lower values and/or higher redshifts than previously accessible, opening the opportunity of definitively testing the CDM hypothesis.

We already have abundant evidence that the simple collisionness, particle-like, cold dark matter paradigm doesn't work. 

Warm dark matter, self-interacting dark matter, wave-like dark matter (to some extent warm dark matter, axion-like dark matter, and fuzzy dark matter), and phase shifting dark matter are the communities main responses to this data.

• Is dark matter production in the early Universe thermal?

The observed relic abundance of dark matter can be explained through a thermal freeze-out mechanism. In this picture, dark matter is kept in thermal equilibrium with the photon bath at high temperatures through weak annihilation processes. Once dark matter becomes non-relativistic, dark matter is still allowed to annihilate, but the reverse process is kinematically forbidden. The continued annihilation of dark matter causes its comoving number density to be Boltzmann suppressed, until it freezes out due to Hubble expansion overcoming the annihilation rate. This process sets the present-day dark matter abundance. 

Importantly, the predicted abundance is sensitive to the detailed dark matter physics, including its particle mass as well as its specific interactions with the Standard Model. Weakly Interacting Massive Particles (WIMPs) provide a classic example of the freeze-out paradigm. In this case, a O(GeV–TeV) mass particle that is weakly interacting yields the correct relic abundance. 

As we will demonstrate, upcoming astrophysical surveys will have the opportunity to definitively test key aspects of the WIMP hypothesis by searching for the rare dark matter annihilation and decay products that arise from the same interactions that set its abundance in the early Universe. A combination of improved instruments and the so-far non-observation of WIMPs has also led to the exploration of probing dark matter candidates that are lighter or heavier than the canonical WIMP window, and which often have a non-thermal origin in the early Universe. This broadening of the possible dark matter candidates that one can search for in indirect detection will continue to be driven by the theory community.

WIMP dark matter is largely ruled out. Candidates lighter than GeV masses are more promising than heavier ones for reasons such as galaxy dynamics. The thermal approach is also muddled by the fact that we really don't understand baryogenesis or leptogenesis either. 

• Is dark matter fundamentally wave-like or particle-like?

Model-independent arguments that rely on the phase-space packing of dark matter in galaxies have been used to set generic bounds on its minimum allowed mass. In particular, a fermionic dark matter candidate can have a minimum mass of ∼ keV, while a bosonic candidate can have a minimum mass of ∼ 10−23 eV. Moreover, when the dark matter mass is much less than ∼ eV, its number density in a galaxy is so large that it can effectively be treated as a classical field. 

Oftentimes referred to as “axions” or “axion-like particles” (ALPs), these ultra-light bosonic states can have distinctive signatures due to their wave-like nature. The QCD axion, originally introduced to address the strong CP problem, is a particularly well-motivated dark matter candidate for which there are clear mechanisms for how to generate the correct abundance today. In this framework, the axion mass and coupling are fundamentally related to each other through the symmetry-breaking scale of the theory As we show, upcoming searches for astrophysical axions will have the sensitivity reach to probe highly-motivated mass ranges for the QCD axion.

Wave-like dark matter means keV scale warm dark matter if it is a fermion, and much, much lighter dark matter bosons if it is bosonic. Since gravity based theories are equivalent to theories transmitted by bosons, the line between wave-like bosonic dark matter and gravity based theories can be thin. Also, keep in mind that even gravitons, while they would have zero rest mass in the vanilla version, would still have non-zero mass-energy that gravitates on a scale comparable to axion-like dark matter bosons. The QCD axion, by the way, is not real and is an ill motivated theory. 

• Is there a dark sector containing other new particles and/or forces?

In a generic and well-motivated theory framework, dark matter can exist in a “dark sector” that communicates with the Standard Model through specific portal interactions. 

Within the dark sector, there can be multiple new states, as well as new forces that mediate interactions between the dark particles. Recent theory work has demonstrated classes of dark sector models that yield the correct dark matter abundance, oftentimes for lower dark matter masses than expected for WIMPs. Dark sector models can lead to a rich phenomenology for both astrophysical and terrestrial dark matter searches, as we will discuss. Two properties of the dark sector where upcoming astrophysical surveys will be able to make decisive statements are the presence of self interactions between dark matter particles and new light degrees of freedom.

Self-interacting dark matter theories are fifth-force theories that no longer have an edge over modified gravity theories in Occam's Razor. 

The number of degrees of freedom in dark matter phenomena is very low as we know from MOND-like theories that manage with just one fixed parameter for a wide range of applicability and from the scaling relation in galaxy clusters that implicates only one more, and also from earlier failures to multi-type dark matter models (citations to which I have not be able to locate again even though I clearly recall these early papers).

• How will the development of numerical methods progress dark matter searches?

Given the sheer volume and complexity of data expected from astrophysical surveys in the upcoming decade, the development of effective observational and data analysis strategies is imperative. Novel machine learning and statistical tools will play an important role in maximizing the utility of these datasets. In particular, scalable inference techniques and deep learning methods have the potential to open new dark matter discovery potential across several frontiers. 

Another critical numerical component to harness the anticipated flood of astrophysical data in the next decade is the further development of cosmological and zoom-in simulations needed to interpret the survey results. We will comment on how such simulations are essential for understanding the implications of particular dark matter models on small-scale structure formation.

There really is an anticipated flood of new astrophysical data in the next decade, which will ground and bound BSM theories of dark matter, to either a null set, or a very narrow range of parameters for far fewer theories.

Also, the paper acknowledges that dark matter models currently fail massively with respect to small-scale structure formation. These theorists take it as an article of faith that better simulations will reveal a dark matter particle theory answer without much of a good basis to believe that. This is not going to come naturally or easily, if it is possible at all.

Also worth noting is the Windchime experiment. This would use tiny mechanical "wind chimes" to directly detect the gravitational effects of individual dark matter particles, overcoming the direct detection barriers associated with dark matter particle types that don't interact with ordinary matter via the Standard Model forces, a very vanilla dark matter hypothesis:

The absence of clear signals from particle dark matter in direct detection experiments motivates new approaches in disparate regions of viable parameter space. In this Snowmass white paper, we outline the Windchime project, a program to build a large array of quantum-enhanced mechanical sensors. The ultimate aim is to build a detector capable of searching for Planck mass-scale dark matter purely through its gravitational coupling to ordinary matter. In the shorter term, we aim to search for a number of other physics targets, especially some ultralight dark matter candidates. Here, we discuss the basic design, open R&D challenges and opportunities, current experimental efforts, and both short- and long-term physics targets of the Windchime project.