For the flat homogeneous universe
ds2 = dτ2 - a(τ)2( dx2+ dy2+ dz2)
we obtain the equations
|3(a' ⁄ a)2||=||-Υa-6 + 3Ξa-2 + Λ + ε|
|2(a" ⁄ a) + (a' ⁄ a)2||=||+Υa-6 + Ξa-2 + Λ - kε|
The Υ-term instead influences the early universe and the gravitational collapse. For the dark matter discussion, this term may be ignored.
The Ξ-term defines a candidate for dark matter. It seems that this term may be used to explain an essential portion of the dark matter (but not all).
GET has not been compared with dark matter observations directly. Nonetheless, we can compare the influence of the Ξ-term roughly with the influence of other, already considered, candidates for dark matter. This gives at least some indirect evidence.
Thus, let's consider existing dark matter theories from point of view of the question if their influence is close to or may be distinguished from the influence of the Ξ-term.
First, there is cold dark matter (CDM). It seems accepted that this theory explains an essential part, but not all of the dark matter. Cold dark matter is not homogeneous, it has higher concentration around galaxies. This allows to distinguish this type of dark matter from other types.
Instead, hot dark matter is homogeneously distributed. This is similar to the Ξ-term.
The difference between hot dark matter and our cosmological term is a different k in the formula for the pressure p = kε. Our term behaves like hot dark matter with p = -1⁄3 ε, while usually matter behaves like dust, that means, p=0. This difference has an influence on the age of the universe: For p=0 the expansion decreases, for p = -1⁄3 ε expansion remains constant.
Another portion of the "dark matter" problem seems to be related with Einstein's cosmological constant Λ. This term, similar to the Ξ-term, increases the age of the universe compared with usual matter.
Different from our term, Einstein's constant increases the expansion of the universe. This property allows independent measurement of this parameter. Current observation suggests a negative value of Einstein's cosmological constant.
This term also does not influence the early universe — this also allows to distinguish it from the influence of our term.
A curvature has a similar influence on the age of the universe as the Ξ-term.
This makes it hard to distinguish our term from the influence of curvature. Nonetheless, the consideration of the inhomogeneities of the background radiation leads to an independent measurement of the curvature. It gives, with good accuracy, a flat universe. Because of this, the curvature term is considered of no importance for the evolution of the universe.
Thus, the Ξ-term has a lot of common properties with many of the current candidates for dark matter. Thus, to propose it as a candidate for some portion of dark matter seems a reasonable hypothesis.
For all candidates which have properties similar to the Ξ-term observation seems to require positive values. Thus, if we use the Ξ-term to explain the same effect, this seems to indicate Ξ > 0.