We study the dynamics of a strongly interacting bosonic quantum gas in an optical lattice potential under the effect of a dissipative environment. We show that the interplay between the dissipative process and the Hamiltonian evolution leads to an unconventional dynamical behavior of local number fluctuations. In particular we show, both analytically and numerically, the emergence of an anomalous diffusive evolution in configuration space at short times and, at long times, an unconventional dynamics dominated by rare events. Such rare events, common in disordered and frustrated systems, are due here to strong interactions. This complex two-stage dynamics reveals information on the level structure of the strongly interacting gas.PACS numbers: 05.70. Ln, 03.75.Kk, 37.10.Jk, Unconventional, non-exponential, relaxation dynamics of a perturbed system towards equilibrium has attracted a lot of interest over decades. Already in 1847, Kohlrausch [1] observed a stretched exponential decay in time t, i.e. e −(t/t0) α with α ∈ (0, 1) and t 0 a positive constant, of the discharge of capacitors fabricated from glasses. Since then, such a decay has been observed in many systems such as molecules and polymers [2,3], spin glasses [4,5], nano-sized magnetic particles [6], and certainly amorphous silicon [7,8].A broad variety of theoretical approaches has been developed to explain the mechanism of this unconventional relaxation dynamics [8][9][10][11]. In many of these approaches, e.g. the treatment of the Griffiths phase in disordered spin systems [12], rare configurations have been identified to play a key role. These configurations have an exponentially small probability to occur, and therefore contribute minimally to the short-time dynamics. However, because their relaxation time scale is very long, these rare configurations can dominate the long time evolution. Rare configurations play an important role in the relaxation dynamics of glasses, where they give rise to stretched exponential decays. We will thus refer to this dynamics induced by rare events as 'glass-like' in the following.In this work, we uncover that also in a quantum many body systems, as the Bose-Hubbard model, the dissipative coupling to a Markovian, i.e. memory-less, environment can cause glass-like dynamics. We show that the long time behaviour in these systems can be dominated by rare configurations. These rare configurations are characterized by a large number of atoms occupying a single lattice site. Increasing the number of atoms on the largely occupied site is associated to a long time scale, since the energetic cost of modifying this kind of configurations is very large. Due to this long time scale, these rare configurations dominate the long time dynamics inducing an unconventional dynamics of stretched exponential form as shown, for the case of local number fluctuations κ = n 2 j − n j 2 (wheren j is the number operator of atoms on site j) in Fig. 1. Additionally, the glass-like dynamics is preceded by an algebraic relaxation process due to the int...