  When the Coulomb repulsion between electrons dominates over their kinetic
energy, electrons in two dimensional systems were predicted to spontaneously
break continuous translation symmetry and form a quantum crystal. Efforts to
observe this elusive state of matter, termed a Wigner crystal (WC), in two
dimensional extended systems have primarily focused on electrons confined to a
single Landau level at high magnetic fields, but have not provided a conclusive
experimental signature of the emerging charge order. Here, we use optical
spectroscopy to demonstrate that electrons in a pristine monolayer
semiconductor with density $ \lesssim 3 \cdot 10^{11}$ cm$^{-2}$ form a WC. The
interactions between resonantly injected excitons and electrons arranged in a
periodic lattice modify the exciton band structure so that it exhibits a new
umklapp resonance, heralding the presence of charge order. Remarkably, the
combination of a relatively high electron mass and reduced dielectric screening
allows us to observe an electronic WC state even in the absence of magnetic
field. The tentative phase diagram obtained from our Hartree-Fock calculations
provides an explanation of the striking experimental signatures obtained up to
$B = 16$ T. Our findings demonstrate that charge-tunable transition metal
dichalcogenide (TMD) monolayers enable the investigation of previously
uncharted territory for many-body physics where interaction energy dominates
over kinetic energy, even in the absence of a moire potential or external
fields.
