  We consider the implications of an ultra-light fermionic dark matter
candidate that carries baryon number. This naturally arises if dark matter has
a small charge under standard model baryon number whilst having an asymmetry
equal and opposite to that in the visible universe. A prototypical model is a
theory of dark baryons of a non-Abelian gauge group, i.e., a dark Quantum
Chromo-Dynamics (QCD). For sub-eV dark baryon masses, the inner region of dark
matter halos is naturally at 'nuclear density', allowing for the formation of
exotic states of matter, akin to neutron stars. The Tremaine-Gunn lower bound
on the mass of fermionic dark matter, i.e., the dark baryons, is violated by
the strong short-range self-interactions, cooling via emission of light dark
pions, and the Cooper pairing of dark quarks that occurs at densities that are
high relative to the (ultra-low) dark QCD scale. We develop the astrophysics of
these STrongly-interacting Ultra-light Millicharged Particles (STUMPs)
utilizing the equation of state of dense quark matter, and find halo cores
consistent with observations of dwarf galaxies. These cores are prevented from
core-collapse by pressure of the 'neutron star', which suggests ultra-light
dark QCD as a resolution to core-cusp problem of collisionless cold dark
matter. The model is distinguished from ultra-light bosonic dark matter through
direct detection and collider signatures, as well as by phenomena associated
with superconductivity, such as Andreev reflection and superconducting
vortices.
