  We study the superposition of primordial massive particles and compute the
associated decoherence time scale in the radiation dominated universe. We
observe that for lighter primordial particles with masses up to
$10^7\,\rm{kg}$, the corresponding decoherence time scale is significantly
larger than the age of the observable universe, demonstrating that a primordial
particle would persist in a pure quantum state, with its wavefunction spreading
freely. For heavier particles, they can still be in a quantum state while their
position uncertainties are limited by the wavelength of background photons. We
then discuss three observational signatures that may arise from a quantum
superposition of primordial particles such as primordial black holes and other
heavy dark matter candidates, namely, interference effects due to
superpositions of the metric, transition lines in the gravitational wave
spectrum due to gravitationally bound states indicating the existence of
gravitons, and witnesses of quantum entanglement between massive particles and
of the gravitational field.
