Cloud 9 and the Limits of Modified Gravity: A Discriminating Test of History-Dependent Dark Matter

Recent observations identify Cloud 9 (M94-CL9), a compact, starless neutral-hydrogen cloud in the vicinity of the spiral galaxy M94, as a leading candidate for a Reionization-Limited H I Cloud (RELHIC): a primordial low-mass halo that retained gas yet never formed stars. Under standard hydrostatic confinement modeling, Cloud 9 implies an extreme dynamical-to-baryonic mass ratio of order ~5000:1, among the most dark-matter-dominated systems inferred to date. We show that Modified Newtonian Dynamics (MOND) struggles to reproduce this regime: in the deep-MOND limit, a naive isolated estimate yields ~40:1, while inclusion of the External Field Effect (EFE) does not generically raise the predicted discrepancy to the required level for an object embedded in M94's gravitational field. Standard ΛCDM can accommodate Cloud 9 as an extreme baryon-depleted primordial mini-halo within the predicted RELHIC population, but this explanation is largely descriptive and delegates the observed diversity of low-mass dark matter content to multiple distinct formation pathways. We then outline the Temporal Echo hypothesis, in which apparent dark matter content reflects integrated dynamical history through a memory kernel with acceleration-dependent decay. In this picture, ancient, dynamically quiet systems deep in the low-acceleration regime naturally accumulate long-lived "laminar wakes," allowing extreme effective mass ratios without requiring a distinct mechanism for each observational class. Cloud 9 thus provides a discriminating testbed for history-dependent models of apparent dark matter.