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QCD and Dark Matter
The nature of dark matter remains a central problem in cosmology. A compelling possibility is that dark matter is macroscopic, consisting of composite objects formed in the early Universe.
The QCD-AQN framework proposes a well-motivated scenario in which dark matter is composed of dense aggregates of quark and antiquark matter stabilised by axion domain walls. The framework proposes a unified explanation for both dark matter and the observed matter antimatter asymmetry.
We propose here a lecture that was originally delivered at the 2025 Les Houches Summer School and published in SciPost Physics Lecture Notes. In this graduate level course, particular emphasis is placed on existing observational constraints and on observational tests.
On this website, we offer a modified version, updated annually to incorporate our team’s latest findings.
Table of content
1 Introduction
2 A critical perspective on model-independent constraints
2.1 Where is dark matter ?
2.2 How dark is dark matter ?
2.3 Cold dark matter
2.4 Collisionless dark matter
2.5 Dark matter interactions with baryons and photons
2.6 Observational constrains from Big Bang Nucleosynthesis
2.7 An overlooked hypothesis: rare dark matter
2.8 Bayesian perspectives on dark-matter models
3 Confronting QCD-AQN to observations
3.1 AQNs structure and properties
3.2 Cosmological behaviour of AQNs: cold and collisionless
3.3 Radiative signatures of AQNs in dilute environments
3.4 Observations in dilute environments
3.5 ¯AQNs interactions with dense objects
3.6 Upcoming tests for the QCD-AQN framework
3.7 Prediction: the dark glow
Note about Section 3: A comprehensive numerical model of AQNs is under development, enabling detailed calculations of their interactions with both dilute and dense cosmic environments and reducing reliance on simplifying approximations. Previous comparisons with observations have been reassessed within this framework, with no major discrepancies identified. Section 3 will be updated once this work is completed.
4 AQNs in the early Universe
4.1 AQN formation
4.2 Matter-antimatter asymmetry
4.3 AQNs remarkable stability
4.4 Photon to baryon ratio, η
4.5 Primordial 7Li abundance
4.6 AQN mass distribution
4.7 Two inflation scenarios
4.8 Broadening the dark-matter paradigm
5 The QCD axion
5.1 QCD axion mass range
5.2 Other types of axions
5.3 Current and upcoming experiments
5.4 The importance of inflation in axion cosmology
5.5 Dark matter as a two-component system