In ΛCDM the universe contains about five times as much dark matter as ordinary baryonic matter: Planck 2018 gives Omega_c h^2 = 0.120 against Omega_b h^2 = 0.0224, a ratio close to 5.4. The problem is that the model assigns these two components completely unrelated origins. The baryon density is set by baryogenesis, an out-of-equilibrium process whose asymmetry parameter eta_B of about 6 x 10^-10 is itself unexplained, while the cold dark matter density is set by an independent mechanism such as thermal freeze-out of a WIMP or misalignment production of axions. Two abundances produced by physically unconnected processes in the early universe could naturally differ by ten or twenty orders of magnitude; instead they agree to within a factor of five.
ΛCDM offers no mechanism connecting the two sectors, so the order-one ratio must be read as a coincidence, and coincidences of order one between unrelated relics are exactly what effective field theory reasoning says not to expect. The model parameter space allows Omega_dm/Omega_b anywhere from much less than one to astronomically large, and nothing in the physics singles out five. Proposed remedies all import new structure: asymmetric dark matter ties a conserved dark number density to the baryon asymmetry but then needs the dark particle mass tuned to about 5 GeV; dark QCD posits an entire mirror strong sector whose confinement scale must land close to Lambda_QCD; relaxation mechanisms add dynamical fields that scan the densities. Each buys the ratio at the price of a new sector built specifically to produce it.
The coincidence deepens when combined with the WIMP null results. LUX-ZEPLIN excludes the canonical WIMP across its preferred mass range, the LHC finds no supersymmetric partners below several TeV, and decades of direct, indirect, and collider searches have produced no dark matter particle. The standard model of cosmology therefore rests on a particle species that has never been detected, whose abundance happens to sit within a factor of five of the baryons for no stated reason. Whatever dark matter is, the Omega_dm/Omega_b ratio looks less like an accident and more like a clue the model cannot read.