* "Phenomenon" is broad: It describes something observable, unusual, or significant. An antonym would depend on which aspect you're focusing on.
* No exact opposite: You can't say something is "un-phenomenal" in the same way you can say something is "un-happy."
Here are some possible alternatives, depending on your meaning:
* For "observable": Invisible, hidden, unnoticeable
* For "unusual": Ordinary, commonplace, expected
* For "significant": Insignificant, trivial, minor
* For "event": Non-event, everyday occurrence
Example:
* Instead of "The aurora borealis is a fascinating phenomenon," you could say:
* "The night sky is usually quite ordinary."
* "There was nothing remarkable about the weather that night."
The best antonym depends on the specific context of your writing.