What about gravity?
Gravity is weird. It is clearly one of the fundamental interactions, but the Standard
Model cannot satisfactorily explain it. This is one of those
major unanswered problems in physics today.
In addition, the gravity force carrier particle has not been found. Such
a particle, however, is predicted to exist and may someday be found: the graviton.
Fortunately, the effects of gravity are extremely tiny in most particle physics
situations compared to the
other three interactions, so theory and experiment can be compared without including
gravity in the calculations. Thus, the Standard Model works without explaining
(I still don't get it.)