Merging clusters separate their ingredients, and the separations have become a battleground. In the Bullet Cluster, the X-ray-emitting gas, holding most of the baryonic mass, lags at the collision midpoint where ram pressure stalled it, while the gravitational lensing mass races ahead with the galaxies: a 720 kiloparsec offset between gas and gravity (Clowe et al. 2006) canonized as the direct empirical proof of collisionless dark matter. The exhibit cuts both ways on inspection. The same offset statistics now constrain any dark sector: self-interaction bounds from offset distributions squeeze SIDM solutions to the small-scale problems; the Bullet's own infall velocity sits in the improbable tail of ΛCDM pairwise kinematics, kin to El Gordo's; and the anomaly file has grown entries the collisionless picture finds awkward, Abell 520's "dark core" where lensing mass sits with the gas and without bright galaxies, dissociated systems whose offsets and timings strain simple ballistic reconstructions.
The deeper methodological point is rarely stated: the offset demonstrates that the gravitating component moves like the galaxies (collisionless) and not like the gas, which any framework reproducing galaxy-coincident gravity passes automatically; the exhibit discriminates against modified-inertia theories whose force tracks the dominant baryonic mass (the gas), but not against frameworks where the effective gravitating mass rides with the galaxy ensemble. The Bullet thereby tests less than its reputation claims, while its velocity and its anomalous siblings test ΛCDM more than advertised.
The standing is an icon under audit: offset catalogs are growing through Euclid and JWST lensing, self-interaction bounds tighten, the anomalous systems resist clean reconstruction, and the exhibit's interpretive weight increasingly depends on which framework is asked to carry it.