how i think about the speed of gravity: if you have a particle accelerator, then you can create and destroy mass. If there is a sensitive accelerometer, then it could detect the changes in mass instantly and make ftl information transfer possible. Since ftl information transfer isn't possible afawk, then gravity must have limited speed.
Also dark matter does probably exist. besides explaining the rotational curves of galaxies, it also affects galaxy clusters in collisions; and we have directly seen dark matter and normal matter being seperated (normal matter is slowed by friction while dark matter is not). since light and gravity travel at the same speed, our methods of gravitational lensing and observation of the electromagnetic spectrum must be viewing the cluster at the same point in time.
also my computer restarted half way through this post and it still had it saved when i restarted. nice.