hoola, I think I see what's confusing you here: the energy in a capacitor is in the electric field created by the charge, not the charge itself. A dielectric is an insulator so no charge can flow in or out of it to build up. That includes a vacuum as a dielectric, which has the baseline permittivity (ε0, 8.85 pF/m). Material dielectrics are compared to ε0 for relative permittivity. In those, the electrons are slightly displaced by the field, thereby partially counteracting it and thus increasing the capacitance.