Cosmological backreaction suggests a link between structure formation and the expansion history of the Universe .
In order to quantitatively examine this connection , we dynamically investigate a volume partition of the Universe into over– and underdense regions .
This allows us to trace structure formation using the volume fraction of the overdense regions \lambda _ { { \cal M } } as its characterizing parameter .
Employing results from cosmological perturbation theory and extrapolating the leading mode into the nonlinear regime , we construct a three–parameter model for the effective cosmic expansion history , involving \lambda _ { { \cal M } _ { 0 } } , the matter density \Omega _ { m } ^ { { \cal D } _ { 0 } } , and the Hubble rate H _ { { \cal D } _ { 0 } } of today ’ s Universe .
Taking standard values for \Omega _ { m } ^ { { \cal D } _ { 0 } } and H _ { { \cal D } _ { 0 } } as well as a reasonable value for \lambda _ { { \cal M } _ { 0 } } , that we derive from N –body simulations , we determine the corresponding amounts of backreaction and spatial curvature .
We find that the obtained values that are sufficient to generate today ’ s structure also lead to a \Lambda CDM–like behavior of the scale factor , parametrized by the same parameters \Omega _ { m } ^ { { \cal D } _ { 0 } } and H _ { { \cal D } _ { 0 } } , but without a cosmological constant .
However , the temporal behavior of \lambda _ { { \cal M } } does not faithfully reproduce the structure formation history .
Surprisingly , however , the model matches with structure formation with the assumption of a low matter content , \Omega _ { m } ^ { { \cal D } _ { 0 } } \approx 3 \% , a result that hints to a different interpretation of part of the backreaction effect as kinematical Dark Matter .

A complementary investigation assumes the \Lambda CDM fit–model for the evolution of the global scale factor by imposing a global replacement of the cosmological constant through backreaction , and also supposes that a Newtonian simulation of structure formation provides the correct volume partition into over– and underdense regions .
From these assumptions we derive the corresponding evolution laws for backreaction and spatial curvature on the partitioned domains .
We find the correct scaling limit predicted by perturbation theory , which allows us to rederive higher–order results from perturbation theory on the evolution laws for backreaction and curvature analytically .
This strong backreaction scenario can explain structure formation and Dark Energy simultaneously .

We conclude that these results represent a conceptually appealing approach towards a solution of the Dark Energy and coincidence problems .
Open problems are the still too large amplitude of initial perturbations that are required for the scenarios proposed , and the role of Dark Matter that may be partially taken by backreaction effects .
Both drawbacks point to the need of a reinterpretation of observational data in the new framework .