.'s
In a paper whose title asks "how rare" is this thing, Eric Hayashi et al. use the millennium simulation to show that this object's velocity is matched by only one out of every hundred subhalos. Observations of this object were corroborated by viewing MACS J0025, as well as Abell 520, which is nicknamed the "train wreck." The smaller of this structure's two constituents passed through the larger one around 150 million years ago and created a gigantic shockwave on its right side. Zaritsky et al.'s observations of this body showed that its center of mass was offset from its center of baryonic mass at a significance of eight sigma; that research team created a weak gravitational lensing map of this body and is among the best available evidence of dark matter, refuting Mordecai Milgrom's MOND theory yet again. For 10 points, name this aggregation formally called 1E 0657-558, a cluster that actually consists of two colliding clusters of galaxies.
In a paper whose title asks "how rare" is this thing, Eric Hayashi et al. use the millennium simulation to show that this object's velocity is matched by only one out of every hundred subhalos.
Bullet cluster [or 1E 0657-558 before mention]