Are we sure about the existence of virtual particles?
Virtual particles definitely exist as mathematical factors in a calculation via perturbation-series expansion, of the interaction probabilities of real particles. Whether they exist as concrete physical entities in an ontological sense is a matter of interpretation. (as are many ontological questions in quantum physics)
The usual calculation of the Casimir effect can be taken as an evidence (though not proof) for the existence of the zero-point energy. This is not the same as virtual particles.But, couldn't Casmir effect and Lamb shift be considered as strong evidence for the existence of virtual particles?