Underscreening is a collective term for charge correlations in electrolytes
decaying slower than the Debye length. Anomalous underscreening refers to
phenomenology that cannot be attributed alone to steric interactions.
Experiments with concentrated electrolytes and ionic fluids report anomalous
underscreening which so far has not been observed in simulation. We present
Molecular Dynamics simulation results exhibiting anomalous underscreening that
can be connected to cluster formation. A theory which accounts for ion pairing
confirms the trend. Our results challenge the classic understanding of dense
electrolytes impacting the design of technologies for energy storage and
conversion