  We report on the observation of negative electronic compressibility in
twisted bilayer graphene for Fermi energies close to insulating states. To
observe this negative compressibility, we take advantage of naturally occurring
twist angle domains that emerge during the fabrication of the samples, leading
to the formation of charge islands. We accurately measure their capacitance
using Coulomb oscillations, from which we infer the compressibility of the
electron gas. Notably, we not only observe the negative electronic
compressibility near correlated insulating states at integer filling, but also
prominently near the band insulating state at full filling, located at the
edges of both the flat- and remote bands. Furthermore, the individual twist
angle domains yield a well-defined carrier density, enabling us to quantify the
strength of electronic interactions and verify the theoretical prediction that
the inverse negative capacitance contribution is proportional to the average
distance between the charge carriers. A detailed analysis of our findings
suggests that Wigner crystallization is the most likely explanation for the
observed negative electronic compressibility.
