Just a little info, as I have gone to a number of lectures on dark matter. It is basically matter that we know HAS to be there, but can not see or detect. Basically we know nothing about it other than the fact that it has to exist for the laws of physics to be correct. Dark matter can be observed when two galaxies collide. I could go on, but it is late =P.
As far as light speed travel. While Einstein was the one to say you can not travel faster than the speed of light, he was also the same one who discovered the basic principle of teleportation. Teleportation consists of two atoms being "linked" together so that what ever happens to one atom happens to the other one at the same time, no matter the distance. This confused Einstein because that means the information traveled faster than light. Theoretically it would be possible to travel faster than the speed of light. If you are in space with no masses around you (aka no gravitational forces), and started to propel a space craft, it would eventually reach a speed right below the speed of light, then reach speed of light, and one more pound of thrust is all it takes to propel you faster than the speed of light. This would be possible, in theory, because no object has a terminal velocity in space.
Another theory is that once you reach the speed of light, time effectively stops.