Layman here, please excuse my ignorance. I believe to understand the basics of SR, GR, cosmic expansion, etc but questions come to mind now and then. As I understand, dark energy, the cosmological constant, vacuum energy, whatever you may call it, remains a constant per space volume unit. Empty space has an intrinsic tendency to expand. Mass-energy in space has the opposite effect, slowing down this expansion. If the mass-energy density is higher than the cosmological constant, space will contract. If it is lower, space expansion will continue at an accelerating rate. So far so good. What I do not understand is the "equilibrium" situation of a flat universe, in which the mass-energy contents of the universe exactly balances the rate of expansion. Frequently I have read that this results in a universe which keeps expanding forever but at a fixed rate, neither accelerating nor slowing down. I do not understand why this "equilibrium" should not be a space which does neither expand not contract, a static spatial extension where neither expansion nor contraction can beat each other. If space keeps on expanding (at a non-accelerating, not-decelerating rate as it is said for a flat universe), the mass-energy density per space volume unit should decrease. Dark energy remains constant per space volume unit, while mass-energy density per space volume unit would decrease as space expands. Intuitively this would mean that per space volume unit, the rate of dark energy to mass-energy would be constantly increasing, boosting space expansion to a faster and faster rate. I do not understand why such an expanding space without mass-energy being added can remain not-accelerating expanding. Where is the flaw in my understanding? Thanks for the answers!