How does this constrain dark matter? In cold dark matter (CDM), clusters have cuspy, steep central potentials, and the BCG oscillates rapidly with a very small amplitude. Self-interacting dark matter (SIDM) softens the core, flattening the potential and allowing wider, slower oscillations. The oscillations take place over very long timescales (millions of years) so in real observations we do not measure one system many times to build an offset distribution, but rather observe a single snapshot of many systems, providing an offset distribution which can be used to study σ/m.