  The stable hadronic bound states in a hidden new non-Abelian gauge sector
provide interesting candidates for strongly-interacting Dark Matter (DM). A
particular example are theories in which DM is made up of dark pions which set
the DM relic abundance through self-annihilation. One of the simplest
realizations is $Sp(4)_c$ gauge theory with two Dirac fermions. We discuss its
mesonic multiplets for degenerate and non-degenerate fermions, construct a
low-energy effective theory and present lattice results for the pseudoscalar
mesons and vector mesons.
