  In this work we consider strongly interacting dark matter candidates as
composite states of $N_f=2$ fermions charged under a dark $Sp(4)$ gauge group
in the fundamental representation. We give expressions that allow the
calculation of correlation functions of two pseudo-Nambu-Goldstone-bosons with
lattice field theory and present first results on the scattering phase shift in
the isospin-2 channel in the theory from first principles. We give a lower
limit on the dark matter particle mass by comparing our results with
astrophysical constraints on the cross-section.
