Dark matter axions clumped together into dense, invisible stars that account for half of the missing mass in our universe.
Invisible dark matter behaves like a thin gas that flows through everything without notice. Intense density fluctuations in the early universe compressed this gas into compact spheres called axion stars. Half of all dark matter can exist in these concentrated bundles rather than a diffuse fog. Detection strategies must now shift away from looking for a steady stream of particles. We are waiting for the rare moment when one of these invisible stars passes directly through Earth.
Pre-inflationary QCD axion stars after moduli domination
arXiv · 2605.00103
The growth of adiabatic density perturbations during an era of early matter domination induces $\mathcal{O}(1)$ fluctuations in pre-inflationary QCD axion dark matter across a broad, string-theory-motivated parameter space. Remarkably, at $\Lambda$CDM matter-radiation equality the scale of these perturbations coincides with the quantum Jeans scale, so they collapse to solitonic ``axion stars''. These axion stars have densities up to $10^4\,\mathrm{eV}^4$, and, including their surrounding halos,