In this case, think of the “*hypo*” in hypothesis to mean “narrow in scope” rather than “unsupported by experiments”. This is a bit different from the typical usage of the word.
In Physics, “theories” tend to have broader scope. E.g., theory of relativity, theory of gravity, etc. These are big things that can explain or predict many different kinds of phenomena.
Comparatively, de Broglie’s hypothesis is limited to one aspect of the wave-particle duality, and may be considered *a part* of the larger theory of quantum mechanics.
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