The hot gas in each cluster was slowed by a drag force, similar to air resistance, during the collision. In contrast, the dark matter was not slowed by the impact because it does not interact directly with itself or the gas except through gravity. Therefore, during the collision the dark matter clumps from the two clusters moved ahead of the hot gas, producing the separation of the dark and normal matter seen in the image. If hot gas was the most massive component in the clusters, as proposed by alternative theories of gravity, such an effect would not be seen. Instead, this result shows that dark matter is required.
So, let's see... hot gasses somehow are affected by a "drag force"... in a vacuum that is almost indistinguishable from the rest of interstellar space, say 1 atom per stere(1 cubic meter) to be really generous, but the much more plentiful and gravitationally active "dark matter" does not even collide with other dark matter????
So what is this "Drag force?" Magic? How can the dark matter pass through the area where the "luminous" matter is interacting strongly... without interacting? Magic again?
Keep in mind also that interstellar space is almost as sparsely occupied by stars... with fewer than 1 star per (10 LY)3 {That's a volume of space equal to a cube comprised of 1000 cubes 1 light year on a side!)... and that galaxies in this cluster are even more sparsley distributed... say 1 per 1,000,000 light year cube. So what are the chances that any one atom, any one star, or even any one galaxy would collide with any other like component of these two clusters? Think of two shotguns, A and B, firing at each other from opposite sides... and the pattern of shot intersects at 30 yards... what are the chances that any one of the pellets from A striking B?
Have these guys stopped to think that the "hot gasses" are actually plasmas? That these "hot gasses" are hot because they carry a charge??? Charged gasses (Plasmas) in the laboratory do not act like uncharged gasses... How about that currents flow through every plasma we create here on earth... easily... current flow creates magnetism, electro-magnetism. Remember basic physics... like charges repel, opposites attract? That will change what apparent affects gravity may have. Electro-magnetism's force is 39 orders of magnitude greater than the force of gravity.
What if what the astronomers are seeing is not a luminous matter/dark matter event but only a luminous matter event as two galaxies clusters with different charges interact? Note that if you look carefully in the space between the two magenta areas that filaments span between them... over thousands of light years... filaments similar to those that are seen in almost any plasma demonstration in the laboratory.