Where Is All the Mass in This?

We're seeing, I assume, unless there's some crazy gravitational lensing, you know, distorting time or something, we're seeing basically a snapshot of the aftermath. And as for when this is very distant out there and happened billions of years ago, now it's going to be even more different. So the question we're trying to ask is, where is all the mass in this? If the truth of the universe is that gravity as we knew it was wrong, then what we would expect is that the location of the mass should be where we see all of the gas. But if the truth is that there's more mass, it's just all dark and non interactive, then