It seems another observed galaxy cluster collision has shown the motion of dark matter via lensing. OK, so perhaps this dark matter stuff does actually exist. What was interesting about this observation is that it seems that dark matter not only doesn't interact with normal matter, but seems to barely interact with itself, other than the gravitational effect. So imagine two galaxy clusters colliding, mostly gas friction I suppose, causing the normal matter to crash, or at least change it's course. And then there are these two clouds of dark matter which pass right through the event, and themselves, and I suppose start swinging round back from the other side as gravity pulls them back in. Its like shadow mass.