If I can prove you wrong simply from Wikipedia it must mean you are very wrong. I've not claimed to be an expert. In fact I know very little about astrophysics. And you despite your discussions with this expert you keep mentioning you keep making very basic errors so it's obvious you aren't an expert either.
Furthermore tell me something. Does this CERN expert of your claim the Big Bang Theory is false?
I never claimed the Big Bang didn't happen. You somkin' someting?
And b.t.w. - you never proven me wrong in the first place.
Galaxies and stars are moving away at a certain speed. Now, the dark matter either exerts some constant influence that affects their speed, or starts exerting some influence when condition X is met.
Let's assume a) is correct.
That means that whatever speed/acceleration the planets/galaxies have already takes into account dark matter (since it was exerting some force on the planets from day 1). We can now calculate the speed and accleration/de-acceleration of galaxies, and knowing the gravitic forces, we can relatively accurately predict if the galaxies will move away far enough so that gravity will not be able to pull them back together.
If it's b) and you believe that at some point, before the galaxies move too far away from eachother, the dark matter will suddenly slow them down/pull them back, then you're no better off then believing Merlin will wave a magic wand and pull them back.
There is NO evidence I've seen that dark matter can slow down the expansion of the universe.
b.t.w. - here, from the wiki:
In astronomy and cosmology, dark matter is hypothetical matter that does not interact with the electromagnetic force, but whose presence can be inferred from gravitational effects on visible matter.You are right - we don't know what the hell it is. We don't even know if it is there. But it's a nice explanation for he strange behaviours observed in space.
But since you don't know what it is, then you can't really say what effect it has.