How does this constrain dark matter? In cold dark matter (CDM), clusters have cuspy, steep central potentials — the BCG oscillates rapidly over a small range. Self-interacting dark matter softens the core, flattening the potential and allowing wider, slower oscillations. The automatic observations measure BCG–centre offsets and build a distribution; wider distributions indicate shallower (more cored) potentials, constraining the self-interaction cross-section σ/m.