I wouldn't trust much anything science related from Wikipedia. I have found far too much there that is posted by people that don't have clue about what they were talking about. Dark matter as I have understood it is mass that is so small that it cannot reflect light.
It is the most abundant mass in the universe according to most theories. The concept of it defining whether our universe is flat (will stop expanding), open (will expand forever), or closed (will stop expanding and then crunch on itself) is somewhat pointless now since recent studies show that our universe is not just open but accelerating. If you want to have a good read
The Universe in a Nutshell has a section on it about dark matter and its properties.
Basically it is just the smallest amount of mass that we can "account" for since there seems to be so much missing mass from the universe.
Some more fun dark stuff is Dark Energy that is the proposed antigravity that makes our universe expand faster and faster. It is caused from quantum fluxuations of free space.
Good luck researching.
