Physicist: In quantum mechanics every event is the sum of the possible ways in which that event can happen. These “maybe-there” particles are virtual. Since this menagerie of particles are all necessary to the interaction, but none of them are guaranteed to exist, physicists have deemed them to be “virtual particles”. So far, it’s always been a virtual particle (it’s unlikely to ever be directly observed). A virtual particle is, by definition, a particle that’s involved in an intermediate stage of an event, but that doesn’t interact with the “outside world”.