Well I had to read that last phrase three times before I got it... I don't think you can assume space-time is dark matter. First, we don't know much about dark matter and we only theorise its use. Second dark matter is supposed to be a type of matter that is undetectable and the only thing it does is have a gravitational field. Also, space time isn't something, it's the entire "volume" of our universe, it's not a flat sheet of paper. I think it's hard to explain, but it is not something, it is where everything exists.
I think a good comparaison would be a game. Take a sandbox game and create a completely empty world. The world is now "space time". It's not something, it's somewhere everything else can exist.