  Moir\'e superlattices provide a powerful tool to engineer novel quantum
phenomena in two-dimensional (2D) heterostructures, where the interactions
between the atomically thin layers qualitatively change the electronic band
structure of the superlattice. For example, mini-Dirac points, tunable Mott
insulator states, and the Hofstadter butterfly can emerge in different types of
graphene/boron nitride moir\'e superlattices, while correlated insulating
states and superconductivity have been reported in twisted bilayer graphene
moir\'e superlattices. In addition to their dramatic effects on the single
particle states, moir\'e superlattices were recently predicted to host novel
excited states, such as moir\'e exciton bands. Here we report the first
observation of moir\'e superlattice exciton states in nearly aligned WSe2/WS2
heterostructures. These moir\'e exciton states manifest as multiple emergent
peaks around the original WSe2 A exciton resonance in the absorption spectra,
and they exhibit gate dependences that are distinctly different from that of
the A exciton in WSe2 monolayers and in large-twist-angle WSe2/WS2
heterostructures. The observed phenomena can be described by a theoretical
model where the periodic moir\'e potential is much stronger than the exciton
kinetic energy and creates multiple flat exciton minibands. The moir\'e exciton
bands provide an attractive platform to explore and control novel excited state
of matter, such as topological excitons and a correlated exciton Hubbard model,
in transition metal dichalcogenides.
