We have carried out x-ray diffraction and small-angle x-ray scattering measurements of expanded fluid alkali metal rubidium (Rb) and observed that the nearest neighbor distance decreases despite the fact that the average interatomic distance increases with volume expansion, indicating that the attractive force among ions is enhanced. Such structural change emerges in the density range where the fluid is still metallic. The density range also corresponds with the region where the instability of the electron gas has been theoretically predicted, suggesting the modulation in the charge fluctuations of the electron gas plays an essential role for such enhanced attraction among ions. Motivated by these unusual structural features, we have moved on investigating the electronic state in expanded fluid Rb through synchrotron-based high-resolution Compton scattering experiments. We have succeeded in measuring the Compton profile of fluid Rb using newly developed a high-pressure vessel for the Compton scattering experiments.