It's not the dark matter that causes the accelrated expansion of the universe, it's negative energy
Dark matter is a way to account for the missing mass of the universe. However, since astronomers discovered that the universe is expanding rather then contracting or at steady state (done by studing class 1 supernova's) they have begun looking for the cause of said expansion. Best they can do now in theorise about "dark energy" or "negative energy", particals that only exist for very short durations in normal space, but the interaction of so meny particals is causing the expansion.
Lovely ain't it?
Also means that in the very distant future, the night sky, and indeed space will be very black, since there will be very few light sources other then our own galaxy. But i degress

The milky way and andromeda are heading in a collision course. No Worries mate.
Ain't the Cosmos grand?
I've seen things you people wouldn't believe. Attack ships on fire off the shoulder of Orion. I watched C-beams glitter in the darkness at Tan Hauser Gate. All those moments will be lost in time like tears in rain. Time to die.