  Quantum mechanics allows for states in macroscopic superpositions, but they
ordinarily undergo rapid decoherence due to interactions with their
environment. A system that only interacts gravitationally, such as an
arrangement of dark matter (DM), may exhibit slow decoherence. In this letter,
we compute the decoherence rate of a quantum object in general relativity. For
axion DM in a superposition of the field's phase, we find that DM in the Milky
Way is robust against decoherence, while a spatial superposition is not. This
novel phase behavior may be relevant to direct detection experiments.
