[Begin summary] Gravitational potentials of galaxy clusters are too deep to be caused by the detected baryonic mass and a Newtonian gravitational force law. One proposed explanation of this mystery invokes dominant quantities of non-luminous "dark matter". The other invokes alterations to the particles' dynamical response to the gravitational force law. The actual existence of dark matter can only be confirmed either by laboratory detection or, in an astronomical context, by the discovery of a system in which the observed baryons and the inferred dark matter are spatially segregated. An ongoing galaxy cluster merger is such a system.

Next, assume that stars make up just 1-2% of the total mass [an assumption valid only if dark matter exists – ed.], and that plasma makes up 5-15% of the total mass [also valid only if dark matter exists – ed.]. But during a merger of two clusters, galaxies behave as collisionless particles, while the fluid-like X-ray–emitting intracluster plasma experiences ram pressure. Therefore, in the course of a cluster collision, galaxies spatially decouple from the plasma.

Such an effect is clearly seen in the unique cluster 1E 0657-558. Two galaxy concentrations that correspond to the main cluster and the smaller sub-cluster have moved ahead of their respective plasma clouds that have been slowed by ram pressure. This phenomenon provides an excellent setup for our simple test. In the absence of dark matter, the gravitational potential will trace the dominant visible matter component, which is the X-ray plasma. [Emphasis added – see below.] If, on the other hand, the mass is indeed dominated by collisionless dark matter, the potential will trace the distribution of that component, which is expected to be spatially coincident with the collisionless galaxies. Thus, by deriving a map of the gravitational potential, one can discriminate between these possibilities.

Weak gravitational lensing of background galaxies shows an observed displacement between the bulk of the baryons and the gravitational potential, which proves the presence of dark matter for the most general assumptions regarding the behavior of gravity. [End summary]

However, this argument needs to assume what it is trying to prove, because it must assume the existence of dark matter and make inferences about the light distribution based on that assumption. But the converse is equally true. If we assume no dark matter, then light and gravity are expected to coincide, just as observed, because the plasma contains a relatively minor contributor to overall mass. In most mature galaxies, most of the mass is in already-formed stars, not in gas and dust. And the dominant visible matter component is likewise the stars. Just because the dust becomes X-ray bright during a collision does not make it massive. See the italicized sentence above, where the opposite was assumed. So the conclusion of this paper is invalid because it uses interpretations valid only if dark matter exists to argue that dark matter exists.