Genady

Yes, it would cause a net gravitational attraction and that would cause slowing of the universe expansion.

Not proved, according to this: "In discussions of the cosmological constant, the Casimir effect is often invoked as decisive evidence that the zero-point energies of quantum fields are “real.” On the contrary, Casimir effects can be formulated and Casimir forces can be computed without reference to zero-point energies. They are relativistic, quantum forces between charges and currents. … I have presented an argument that the experimental confirmation of the Casimir effect does not establish the reality of zero-point fluctuations. Casimir forces can be calculated without reference to the vacuum ... . The vacuum-to-vacuum graphs (See Fig. 1) that define the zero-point energy do not enter the calculation of the Casimir force, which instead only involves graphs with external lines. So the concept of zero-point fluctuations is a heuristic and calculational aid in the description of the Casimir effect, but not a necessity." Casimir effect and the quantum vacuum R. L. Jaffe Phys. Rev. D 72, 021301(R) – Published 12 July 2005

If there were much more of it around, it would affect the cosmological expansion. Perhaps it could be detected this way.

The distribution of the DM in space now is very much non-uniform. Since it doesn't clump, was its distribution as non-uniform soon after the BB?

Is it necessarily so? If it could interact electromagnetically by completely reflecting an EM radiation it wouldn't emit a thermal EM radiation, would it?

Many think that it interacts not only via gravity but also weakly, i.e. made of WIMPs.

I don't know why it's called dark, but not interacting electromagnetically makes it rather transparent than dark.

We also know that it was there by the time of "recombination", about 380 000 years after the Big Bang.

We know where it is and how much of it is there. We don't know what it is.

Nice. Except the very last statement. That is a kind of "wishful thinking" fallacy.