We consider a Yang-Mills-Higgs theory with gauge group broken to by a Higgs field in the adjoint representation. We obtain monopole solutions whose magnetic field does not lie in the Cartan Subalgebra. And, since their magnetic field vanishes in the direction of the electromagnetic group, we call them Dark Monopoles. These Dark Monopoles must exist in some Grand Unified Theories (GUTs) without the need to introduce a dark sector. We calculate the general hamiltonian and equations of motion, while we also obtain approximate analytical solutions when and . We show that their mass is given by , where is a monotonically increasing function of , with the lower and upper bounds depending on specific parameters of each possible symmetry breaking. For the particular case of the GUT, we obtain that and . In addition, we give a geometrical interpretation to their non-abelian magnetic charge and we show that our monopole solution has a conserved magnetic current , which is quantized and lies in a non-abelian direction. Finally, we proceed with an asymptotic stability analysis of these Dark Monopole solutions, where we show that there are unstable modes associated with them. We obtain the explicit form of the unstable perturbations and the associated negative-squared eigenfrequencies.