The word "phenomenon" refers to an observable fact or occurrence. As a noun that represents something that exists or happens, it doesn't have a true antonym in the traditional sense. However, we can think of words that are in some ways oppositional in meaning or concept.
For example, "non-occurrence" or "absence" could be seen as contrasting with "phenomenon" to some extent. If a phenomenon is an event or occurrence that is noticed and remarked upon, then the absence of that event or occurrence could be thought of as an opposite. For instance, "The expected phenomenon did not occur; there was only an absence of any significant change."
Another possible contrasting term could be "obscurity" or "invisibility." While a phenomenon is something that is visible or noticeable, something that is in obscurity or is invisible is not a phenomenon in the sense of being an observable occurrence. For example, "The once prominent phenomenon faded into obscurity as time passed."
In conclusion, although there isn't a perfect antonym for "phenomenon," words like "non-occurrence," "absence," "obscurity," or "invisibility" can offer some contrast in meaning and help to think about what is not a phenomenon in relation to what is.