Does Dark Matter Matter?

My questions are the questions of a moron, but you'll have to cut me some slack because I know no astrophysics & the article doesn't address this.

How do they know that the galaxies in question have gone beyond the reach of gravity, and were not beyond it the whole time—that is, independent of the galaxies that collided? Do they have images of this cluster before the collision? I find that unlikely. Is it red shift? If so, how can they measure the red shift in the two dimensions that are orthogonal to our perspective?

You decide.

See, that's what I love about science: my opinion doesn't really matter because the objective reality is out there no matter what my personal opinion is. The same thing hold true for Divine Reality, of course.

My random .signature quote generator picked this morning the quote from Chesterton: "Truth, of course, must of necessity be stranger than fiction, for we have made fiction to suit ourselves."

"Dark matter: see Aether."