that conjectures a large backreaction effect (e.g. [@kolbetal]) has been accompanied both by counter conjectures (e.g. [@wald]) and special solutions of the averaged Einstein equations [@buchert:grgdust] that support the claim that backreaction could be made responsible for the Dark Energy gap (e.g. [@buchert:darkenergy], [@nambu], [@alnes:LTB], [@rasanen:constraints; @rasanen:model], [@curvatureLTB] and references and discussion in [@ellisbuchert]).

In parallel to this discussion, other models have been extensively investigated, most notably quintessence models that invoke the presence of a scalar field source, this scalar field being a standard one or a phantom field (with a negative kinetic energy) [@Wetterich88; @Peebles2003; @Faraoni2005; @Jassal2006] and references in the reviews [@sahnistarobinskii] and [@copeland]. Those models imply the possibility of a scenario featuring a time–dependent cosmological term. Particular properties of the scalar field potential are discussed on phenomenological grounds, which can lead to an acceleration of the regional Universe and so to an explanation of Dark Energy. A further problem in conjunction with modeling a repulsive component in universe models is known as the [*coincidence problem*]{}, i.e. a recent domination of this component is favoured, possibly occuring around the epoch of structure formation. Its solution also motivates the construction of models with a time–dependent cosmological term that needs 