Galaxy cluster centers come in two kinds and almost nothing in between: Cool Core systems whose dense centers radiate furiously and should cool within gigayears, and Non-Cool Core systems with flat, high-entropy centers, a bimodality sharper than any continuous formation process naturally draws (Fabian 2012).
Each population poses its own problem and the dichotomy poses a third. The cool cores embody the venerable cooling flow problem: their radiative losses predict hundreds of solar masses per year of condensing gas that simply is not observed, demanding an AGN feedback loop that reheats the center at precisely the cooling rate, self-regulated over billions of years without ever overshooting into core destruction or undershooting into runaway cooling. The non-cool cores need their high-entropy centers explained, with mergers the assigned mechanism, yet simulations find cool cores stubbornly resilient, disrupted only by the most violent head-on collisions, leaving NCC abundance underproduced (McDonald et al. 2018). And the sharp bimodality itself resists: regulated feedback plus stochastic mergers should populate the middle ground, and the middle ground is nearly empty.
The standing is a two-decade impasse in cluster astrophysics: the most X-ray-luminous objects in the sky divide into two families whose origins, maintenance, and mutual exclusivity all require tuning the standard toolkit was not built to provide.