  Recent theoretical progress indicates that spacetime and gravity emerge
together from the entanglement structure of an underlying microscopic theory.
These ideas are best understood in Anti-de Sitter space, where they rely on the
area law for entanglement entropy. The extension to de Sitter space requires
taking into account the entropy and temperature associated with the
cosmological horizon. Using insights from string theory, black hole physics and
quantum information theory we argue that the positive dark energy leads to a
thermal volume law contribution to the entropy that overtakes the area law
precisely at the cosmological horizon. Due to the competition between area and
volume law entanglement the microscopic de Sitter states do not thermalise at
sub-Hubble scales: they exhibit memory effects in the form of an entropy
displacement caused by matter. The emergent laws of gravity contain an
additional `dark' gravitational force describing the `elastic' response due to
the entropy displacement. We derive an estimate of the strength of this extra
force in terms of the baryonic mass, Newton's constant and the Hubble
acceleration scale a_0 =cH_0, and provide evidence for the fact that this
additional `dark gravity~force' explains the observed phenomena in galaxies and
clusters currently attributed to dark matter.

High Energy Physics - Theory;General Relativity and Quantum Cosmology