Can Galactic Halos Be Made of Axions?

If axions exist, they contribute significantly to the present energy density. These axions have been collisionless and nonrelativistic since their appearance at ∼ 1 GeV. Hence axion density perturbations survive on essentially all scales, and once the axions are dominant, grow and in turn create and amplify baryon fluctuations. Axions can cluster into galactic halos even though the axion mass ≲${10}^{-2\mathrm{}}$ eV. If galactic halo matter is mainly axions, the Peccei-Quinn order parameter ≳${10}^{10}$ GeV.