Why is Space Dark?

The reason why space is dark is a lot more simple than I had expected at first, but that doesn't make it very simple still. Dark Matter is a nonluminous thing that could possibly make up nearly 99% of our universe. Though, we can't see it directly, we can see its effect on the night sky. We know of the existance of dark matter for a very simple reason, it has gravity on normal matter which was evident from small changes in orbits in distant galaxies. We do not know what Dark Matter is really made of or it's effect on itself and normal matter.
Click here to see cool video: http://www.pbs.org/wgbh/nova/sciencenow/0301/01.html