Observations of galaxy clusters, particularly merging systems, reveal spatial offsets between the peak of the X-ray-emitting hot gas distribution and the peak of the gravitational lensing mass distribution, with the lensing peaks typically leading the gas by tens to hundreds of kiloparsecs in the direction of motion. Lambda-CDM interprets these offsets as evidence for collisionless dark matter that passes through the collision unimpeded while the gas experiences ram pressure and hydrodynamic drag, but the model faces challenges in explaining the diversity of offset magnitudes and geometries observed across different systems, the occasional absence of expected offsets in some mergers, and the precise quantitative relationship between collision velocity, gas density, and offset distance (Massey 2015; Harvey 2015). The standard dark matter interpretation also struggles with why some clusters show more complex offset patterns involving multiple lensing peaks or asymmetric gas distributions that don't align simply with the collision-less versus collisional dichotomy, and why the inferred dark matter distributions sometimes show their own substructure that correlates imperfectly with galaxy positions.