  We consider dark matter as Strongly Interacting Massive Particles (SIMPs) in
a hidden sector, thermally decoupled from the Standard Model heat bath. Due to
its strong interactions, the number-changing processes of the SIMP lead to its
thermalization at temperature $T_{\rm{D}}$ different from the visible sector
temperature $T$, and subsequent decoupling as the Universe expands. We study
the evolution of the dark SIMP abundance in detail and find that a hidden SIMP
provides for a consistent framework for self-interacting dark matter.
Thermalization and decoupling of a composite SIMP can be treated within the
domain of validity of chiral perturbation theory unlike the simplest
realizations of the SIMP, where the SIMP is in thermal equilibrium with the
Standard Model.

High Energy Physics - Phenomenology