  Quantum many-body scars are notable as nonthermal states that exist at high
energies. Here, we use attractively interacting dysprosium gases to create scar
states that are stable enough be driven into a strongly nonlinear regime while
retaining their character. We uncover an emergent nonlinear many-body
phenomenon, the effective transmutation of attractive interactions into
repulsive interactions. We measure how the kinetic and total energies evolve
after quenching the confining potential. Although the bare interactions are
attractive, the low-energy degrees of freedom evolve as if they repel each
other: Thus, their kinetic energy paradoxically decreases as the gas is
compressed. The missing ``phantom'' energy is quantified by benchmarking our
experimental results against generalized hydrodynamics calculations. We present
evidence that the missing kinetic energy is stored in very high-momentum modes.
