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A major hurdle for modified gravity theories is to explain the dynamics ofgalaxy clusters. A case is made for a generalized gravitational theory calledScalar-Tensor-Vector-Gravity (STVG) or MOG (modified gravity) to explainmerging cluster dynamics. The paper presents the results of a re-analysis ofthe Bullet Cluster, as well as an analysis of the Train Wreck Cluster in theweak gravitational field limit without dark matter. The King-$\beta$ model isused to fit the X-ray data of both clusters, and the $\kappa$-maps are computedusing the parameters of this fit. The amount of galaxies in the clusters isestimated by subtracting the predicted $\kappa$-map from the $\kappa$-map data.The estimate for the Bullet Cluster is that $14.1\%$ of the cluster is composedof galaxies. For the Train Wreck Cluster, if the Jee et al. data are used,$25.7\%$ of the cluster is composed of galaxies. The baryon matter in thegalaxies and the enhanced strength of gravitation in MOG, shift the lensingpeaks making them offset from the gas. The work demonstrates that thisgeneralized gravitational theory can explain merging cluster dynamics withoutdark matter.

The Train Wreck Cluster Abell 520 and the Bullet Cluster 1E0657-558 in a Generalized Theory of Gravitation