The interdimensional hypothesis proposes that at least some UFO and entity reports originate from a domain outside ordinary human spacetime. Unlike the extraterrestrial hypothesis, it does not require visitors to cross interstellar distances in conventional spacecraft. The phrase covers several different ideas—parallel realities, hidden domains, altered states, or unfamiliar aspects of physics—so it has no single agreed mechanism or test.

Researchers including Jacques Vallée developed versions of the argument after comparing modern UFO stories with older reports of apparitions, fairies, and religious visions. Vallée emphasized absurd, theatrical, and psychologically transformative features that seemed inconsistent with a simple survey mission by off-world travelers. His work shaped the modern hypothesis, but similarity between stories across time can also reflect recurring human cognition, narrative borrowing, or cultural reinterpretation.

The hypothesis is often invoked for abrupt appearances, missing time, telepathy, or reports of interdimensional beings. None of those features independently identifies an extra dimension. Physics does use higher-dimensional spaces and explores speculative cosmologies, yet those models do not predict the reported entities or demonstrate transit between inhabited realities. The leap from a mathematical dimension to an encounter narrative is therefore an inference, not an established scientific result.

As an analytical category, the hypothesis can be compared with extraterrestrials, the ultraterrestrial hypothesis, and psychological or sociological explanations. It is strongest when framed as a question about patterns in testimony and weakest when used to immunize a claim from testing: any unexplained detail can otherwise be assigned to an invisible realm. Progress would require distinctive, repeatable predictions and evidence that discriminates an interdimensional source from error, folklore, deception, unknown natural phenomena, or advanced but ordinary technology.