Bound states dark and bright

We elaborate on the physics of both dark and bright bound states. The first -- dark -- refers to the possibility that the dark matter of our Universe is of composite nature, and in particular we focus on models in which the dark matter is accidentally stable, and that can be embedded in the framework of grand unified theories. We discuss general aspects of both QCD-like and weakly coupled scenario, before turning to a classification of models in which we include a fundamental scalar constituents along with several fermions. We discuss the impact of the addition of the scalar to the phenomenology of composite dark matter. We also address the question of whether it is possible to construct accidentally stable ultraheavy dark matter candidates in this context. The second -- bright -- refers to bound states of electromagnetism, and in particular to true muonium, the yet to be discovered bound state of a muon and its anti-particle. We discuss three proposals for discovering it, at the Da$\Phi$ne collider in Frascati, at the H4 facility at CERN-SPS, and in already collected data at Belle-II, finding that all three show potential for discovery in reasonable timespans.