The DM is moved around by the gravity of the moving galaxy. The DM theory is that each galaxy is surrounded by a huge spherical cloud of DM that (by definition)

The DM is moved around by the gravity of the moving galaxy. The DM theory is that each galaxy is surrounded by a huge spherical cloud of DM that (by definition) only experiences gravitation and is invisible and insubstantial in every other way. When Voyager gained speed from passing near Jupiter, it caused Jupiter to slow down by a tiny amount. Similarly, when galaxies pass through each other, the DM should take a measurable amount of momentum from the galaxies. And, yeah, it's a provocative title, but he believes that his measurements falsify DM theory. It's partly a tussle between experimentalists and theoreticians. Kroupa is arguing that the claims of the theory can be tested, and that it doesn't match observations. The theoreticians have a nice story for the big bang, inflation, baryonic nucleosynthesis and the variations seen in the CMB, and the amount of DM it predicts happens to match precisely the amount of DM that we observe.