Quantum gravity has been a central challenge in theoretical physics. Recently, with the fast development of quantum information science, there is a growing interest in exploring a different parameter space of quantum gravity -- superposition of massive objects and their gravitational effects. We explore gravitational superpositions in the early and late universe. Namely, we reconsider the decoherence of cosmological perturbations during inflation, show that decoherence happens at a faster rate than that of our previous understandings. And we study the position uncertainty of dark matter particles, subject to gravitational decoherence by CMB photons.