When Should the Dark Matter Hypothesis Be Reconsidered? -- How Should a Hypothesis Be Evaluated When Its Concrete Physical Nature Remains Unknown for Nearly a Century? --

This paper examines when and why the dark matter hypothesis should be reconsidered from a methodological perspective. Dark matter has been used for decades to explain a wide range of astronomical observations, yet its concrete physical nature remains unidentified. Numerous candidates, including WIMPs and axions, have been proposed and experimentally investigated. However, the existence of many candidates does not necessarily mean that observations have progressively narrowed the identity of dark matter. In many cases, these candidates are theoretical possibilities constructed to satisfy properties inferred from observations. The paper distinguishes between dark matter as an explanatory hypothesis and dark matter as a specific physical entity existing in nature. It asks whether the prolonged absence of a concrete physical identification should itself become a reason to reconsider not only individual candidates, but also the assumptions underlying the hypothesis. The paper does not claim to disprove dark matter. Instead, it argues that scientific hypotheses should remain open to reconsideration when their central physical interpretation remains unresolved despite sustained investigation.