The mechanisms that translate early childhood education (ECE) programs' initial impacts into longer-term effects are still unknown. Many candidate mediating variables, like cognitive or self-regulation skills, personality traits, or more structural institutional gateways have been proposed. Yet, mediation analyses most often leave much of the total impact on long-term outcomes unaccounted for. This unexplained portion of the effect, so-called "dark matter" of ECE, affords different developmental explanations according to the type of mediators considered and whether the latter are measured proximally or distally to ECE interventions. Leveraging panel data from both experimental and quasi-experimental designs (the Abecedarian early intervention and a sibling fixed effect analysis of Head Start; n = 107 and n = 1,251, respectively), we investigated ECE interventions' patterns of indirect effects on adulthood completed schooling, across multiple mediators. We found some evidence--having estimated observed indirect effects accounting for up to 50 percent of the total impacts--that the "dark matter" effect was neither more nor less prominent upstream than downstream, but rather stable across development. These findings were consistent with the hypotheses that earlier skills contributed to later institutional gateway process; and that pathways running parallel to our chosen mediating bundles, possibly coupling environmental dimensions with behavior, ought to be specified.