Examples of why dark matter can't bounce off ordinary matter?

I recall that there's at least two examples of why dark matter can't bounce off of ordinary matter due to electromagnetic interactions:

1. If dark matter bounced off of ordinary matter, then it bouncing off of gas/plasma clouds would change its distribution and velocities within galaxies to the point where dark matter couldn't account for galactic rotation curves.
2. There's at least one instance where two galaxies passed through each other, and as a result the gas/plasma clouds of the two galaxies merged (or one was stripped away from the other), but the dark matter stayed in each respective galaxies?

Is my understanding correct? If so, could anyone provide citations/links?

via International Skeptics Forum http://ift.tt/2pqhklF