The Bullet Cluster is dark matter's favorite photograph: two galaxy clusters that collided and passed through each other, the hot X-ray gas slowed by ram pressure into shock fronts between them while the weak-lensing mass peaks sailed ahead with the galaxies, a 720 kpc separation between where the baryons are and where the gravity points (Markevitch et al. 2004; Clowe et al. 2006). The standard reading: the lensing mass is collisionless dark matter that passed through unimpeded.
The photograph troubles its owner too. The shock structure implies a collision velocity near 4700 km/s, at or beyond the extreme tail of what hierarchical structure formation produces for cluster pairs at this epoch, improbable enough that the system's existence has itself been wielded as a ΛCDM problem; the cleanness of the gas-lensing separation exceeds what most merger simulations deliver; and the energetics sit uncomfortably with merger statistics in a dark-matter-dominated universe. The system thus cuts both ways: its offset is the particle picture's best exhibit, while its velocity is among that picture's worst embarrassments, and any alternative framework must answer the offset to be taken seriously.
The standing makes the Bullet the benchmark every gravity-side theory is measured against, with the El Gordo collision compounding the velocity problem at higher redshift.