  The nature of pseudogap lies at the heart of strongly-interacting
superconductivity and superfluidity. With known pairing interactions, unitary
Fermi gases provide an ideal testbed to verify whether a pseudogap can arise
from many-body pairing. Here we report the observation of the long-sought
pair-fluctuation-driven pseudogap in homogeneous unitary Fermi gases of
lithium-6 atoms, by precisely measuring the spectral function through
momentum-resolved microwave spectroscopy without the serious effects of
final-state effect. We find a large pseudogap above the superfluid transition.
The inverse pair lifetime exhibits a thermally-activated exponential behavior,
uncovering the microscopic virtual pair breaking and recombination mechanism.
The obtained large, T-independent single-particle scattering rate is comparable
with that set by the Planckian limit. Our findings quantitatively characterize
the pseudogap in strongly-interacting Fermi gases, highlighting the role of
preformed pairing as a precursor to superfluidity.
