  Axions and axion-like particles (ALPs) are a prominent dark matter candidate,
drawing motivation in part from the axiverse of string theory. Axion-like
particles can also arise as composite degrees of freedom of a dark sector, for
example, as dark pions in dark Quantum Chromo-Dynamics. In a dark Standard
Model (SM) wherein all 6 quark flavors are light while the photon is massive,
one finds a rich low-energy spectrum of stable and ultralight particles, in the
form of neutral and charged dark scalars, and complex neutral scalars analogous
to the SM kaon, with mass splittings determined by the mass and charge of the
dark quarks. The model finds a natural portal to the visible sector via kinetic
coupling of the dark and visible photons, and consequent millicharges for dark
matter. The dark matter can be a mixture of all these ultralight bosonic
degrees of freedom, and exhibit both parity-even and parity-odd interactions,
making the theory testable at a wide variety of experiments. In context of dark
QCD with $N_f$ flavors of light quarks, this scenario predicts $N_f^2-1$
ultralight axion-like particles -- effectively an axiverse from dark QCD. This
'$\pi$-axiverse' is consistent with but makes no recourse to string theory, and
is complementary to the conventional string theory axiverse.
