The gravitational natures of phenomena separately attributed to dark matter and dark energy and challenges encountered in identifying such sources motivate enquiry into the capabilities of the field, itself, to generate such phenomena. It is found that, in curvature-free Friedmann-Lemaître-Robertson-Walker and gravitationally perturbed Robertson-Walker spacetimes, gravity has an equation of state parameter w = -1 and negative pressures. Expanding space is proposed as the form of a growing cosmic gravitational field. The gravitational-spatial expansion is locally isobaric. Barotropic gravitational dynamics yield the Hubble-Lemaître law. The expansion results from the induction of gravity by matter, radiation and by itself. Gravitational auto-induction is a dynamical feedback process that produces an isotropic spatial expansion with an invariant Hubble parameter like a ‘cosmological constant’ of density 2H2/κ or, equivalently, of a density parameter of 2/3. The Planck 2018 result is moderately higher at about the 2.5/σ level. A new expression of the Hubble parameter in the late homogeneous universe is obtained. The growth of the field isotropically stretches geodesics. In homogeneous regions, this manifests as the Hubble acceleration of bodies and the redshifting of radiation attributed to dark energy. Geodesics may depend on gravitational energy density that retains its values at comoving locations. In inhomogeneous regions, such retentions lead to similar retentions of circular speeds and deflection angles - geodesic stretching - attributed to clustering dark matter. The baryonic Tully-Fisher relation is explained. Dependence of geodesics on gravitational energy explains tidal interactions as being inertial gravitational processes.