  Analyzes of astrophysical data provide first hints on the self-interactions
of dark matter at low energies. Lattice calculations of dark matter theories
can be used to investigate them, especially in the case of strongly-interacting
dark matter. We consider Sp(4) gauge theory with two fundamental fermions as a
candidate theory. We compute the scattering phase shift for the scattering of
two identical dark pions and determine the parameters of the effective range
expansion. Our exploratory results in the supposedly most common interaction
channel provide a lower limit for the dark matter mass when compared to
astrophysical data. We also provide first benchmarks of velocity-weighted
cross-sections in the relevant non-relativistic domain.
