Virtual particles appear and disappear in the course of interactions between real particles. The general phenomenon of particles appearing and disappearing in interactions happens with both real and virtual particles—but the interactions are such that momentum, charge, etc. are always conserved. So the things you should expect to see persisting in existence aren’t particles themselves, but those conserved properties.

The interactions between real particles involve the appearance and disappearance of virtual particles. In these processes, both real and virtual particles can emerge and vanish, yet the interactions always ensure that properties like momentum and charge remain conserved. Therefore, what you should expect to persist are not the particles themselves, but the conserved properties that define them.

Virtual particles are still only theoretical. I think of them as a mental model of mathematical objects that arise when we try to calculate interactions between real particles.

I know there are some experiments like casimir effect that are attributed to virtual particles. But, it is possible to explain it in other ways.

They are basically an energy credit. They don’t go anywhere. They don’t “come back” in any way.