I tried to come up with a different approach to the galaxy rotation problem.
Can anyone tell me if this makes any sense, has been done before or if I'm just making a dumb mistake.

The basic idea is that galaxies have more space in the middle.

Let's say Dark Matter and Modified Gravity theories will never fully explain why outer objects have such high velocities.
This leaves the possibility that we need to increase the velocity of inner objects.
If we increase the velocity of objects, exponentially, as we go from the edge of a galaxy to it's centre, this should transform the observed velocity curve into the same shape as the predicted velocity curve, just at a different scale.

Velocity, Time and The Laws of Physics seem to have been ruled out as variables, leaving Distance as the next obvious candidate.
Increasing the distance an object travels per orbit, whilst maintaining it's orbital period, will increase it's velocity.

The increase in “proper” space, per observed space, has to increase exponentially (from centre to edge) and the amount of gravity affecting any region is the only match I can think of.
Gravity also seems to increase the density of everything else and we know that “gravity warps space-time”.
So saying that gravity increases the density of space-time seems possible to me.