This should not be too confounding. Suppose you have two galaxies collide. The dark matter will sail right through the other galaxy, affected only by the overall gravity. The stars will almost never hit each other, so the vast majority of them will be affected only by the overall gravity too. The gas and dust will not - dust is subject to radiation pressure, and gas (plasma) magnetic fields. Once the gas and the dark matter become separated, there is no guarantee they will ever get back together. As the paper says :
One of the key tools for studying merging clusters is the comparison among the distributions of the three cluster constituents: galaxies, hot plasma, and dark matter. For example, in merging clusters the intracluster medium suffers from ram pressure and lags behind galaxies and dark matter, which are believed to be effectively collisionless. The contrast between collisional and collisionless components becomes highest when we observe merging clusters at their core pass-through, when both the medium velocity and the effect of ram pressure stripping are largest.
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