Microphysical nature of dark matter

Determine the microphysical underpinnings of cold dark matter by identifying the particle or field content, interactions, and action that account for its gravitational effects and cosmological abundance while remaining consistent with precision cosmological observations.

Background

The authors note that, at cosmological scales, dark matter is effectively described by a pressureless fluid, and many candidate microphysical models (e.g., WIMPs, axions/ultralight scalars, primordial black holes) can reproduce the required bulk behavior. Cosmological probes thus provide limited discriminatory power among microphysical candidates.

Non-cosmological searches (direct and indirect detection, astrophysical signatures) have not yet yielded definitive identifications, and large viable parameter spaces remain. This leads to substantial underdetermination of the microphysical theory of dark matter.

References

We now find ourselves with three open challenges: what are the microphysical underpinnings of inflation, dark matter, and dark energy?

— The Spectre of Underdetermination in Modern Cosmology  (2501.06095 - Ferreira et al., 10 Jan 2025) in Section 4: The spectre of underdetermination

While the $\Lambda$CDM model successfully describes the large-scale distribution of matter, the microphysical nature of DM remains unknown, and different particle physics scenarios predict distinct behaviors on small scales \citep[e.g.,][]{feng:2010,Bozorgnia_2025}.

— Impact of LSST systematics on stellar-stream density fluctuations for dark matter  (2609.10897 - Pélissier et al., 9 Sep 2026) in Section 1, Introduction

Although compelling observational evidence accumulated over the past several decades has established that dark matter constitutes approximately $85\%$ of the matter content of the Universe, its microscopic nature remains unknown and its distribution in the strong-gravity regime surrounding SMBHs remains poorly constrained.

— Probing the Distribution and Nature of Dark Matter Around Supermassive Black Holes from EMRI and IMRI Gravitational Waves  (2608.12540 - Fier et al., 12 Aug 2026) in Section I, Introduction