  Adding energy to a system through transient stirring usually leads to more
disorder. In contrast, point-like vortices in a bounded two-dimensional fluid
are predicted to reorder above a certain energy, forming persistent vortex
clusters. Here we realize experimentally these vortex clusters in a planar
superfluid: a $^{87}$Rb Bose-Einstein condensate confined to an elliptical
geometry. We demonstrate that the clusters persist for long times, maintaining
the superfluid system in a high energy state far from global equilibrium. Our
experiments explore a regime of vortex matter at negative absolute
temperatures, and have relevance to the dynamics of topological defects,
two-dimensional turbulence, and systems such as helium films, nonlinear optical
materials, fermion superfluids, and quark-gluon plasmas.
