What made you believe that dark matter doesn't have any effect on neighboring galaxy?

Secondly, dark matter and dark energy are just placeholder names that we don't know anything about. We know whatever it is, its some sort of matter/energy because that's the closest thing we could compare it to.

Also, consider this: We can only observe less 5% of our universe. And you are claiming that this thing what constitutes more than a quarter of our universe has no effect on the rest of it?

Why do we call it dark matter? Matter? because it interacts with our "visible matter" through gravity. We calculated gravity of all the visible matter and realized that 85% of gravity is still missing. We figured that since gravity comes from matter as it bends the fabric of space-time. Maybe there is another matter that bends the fabric as well but we can't observe it (hence dark).

There are plenty of effects of dark matter that we observe and "galaxy's spiral-arm" thing is one of the many effects of dark matter.