  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 within general relativity,
focusing on superposed metric oscillations; a rare quantum general relativistic
result. 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 impact direct detection experiments.
