The Canis Major overdensity is either the Milky Way's most recently devoured victim or an optical illusion of its own bent disk, and twenty years of debate has not closed the case. Discovered in 2003 as a strong excess of M-giant stars at low Galactic latitude toward Canis Major, about 25,000 light-years from the Sun, it was announced as the core of a disrupting dwarf galaxy, conveniently positioned to be the progenitor of the Monoceros Ring and direct evidence of ongoing accretion feeding the outer disk. The accretion reading gave ΛCDM a showcase: hierarchical assembly caught mid-meal.
The dissenting analyses have steadily gained ground: the Galactic disk in that direction is both warped and flared, and models of the warp reproduce much of the stellar excess without any accreted population; the claimed dwarf's stellar populations, kinematics, and distance spread match the perturbed outer disk better than a bound remnant in several studies; and no convincing tidal-stream connection or distinct orbital family has been isolated. Gaia-era chemo-dynamics has sharpened rather than settled the dispute, with the region's populations reading as predominantly disk-like while localized substructure persists. Either resolution costs the standard model something: if accreted, the bookkeeping of the event (orbit, mass, timing, plus its Monoceros linkage) has resisted consistent reconstruction; if a warp feature, the showcase accretion event evaporates, and the warp itself, large, recent, and dynamically unexplained, takes its place on the anomaly ledger.
The standing is a genuinely unresolved classification problem at the disk's edge, entangled with the Monoceros Ring's origin and the deeper question of what keeps bending the Galaxy's outer disk.