We have measured x-ray diffraction and small-angle x-ray scattering of fluid rubidium by reducing electron density down to the range where the compressibility of the interacting electron gas has been theoretically predicted to become negative. Negative compressibility is closely associated with a negative value of the static dielectric function, which makes the screened Coulomb interaction among like charges overall attractive. It was clearly observed that the interatomic distance decreases in spite of the fact that mean interatomic distance increases with expansion, suggesting that an attractive interaction among like charges, ions in this case, is enhanced. These findings indicate that the observed structural features are evidence of the compressional instability of the 3D electron gas.