Are virtual particles really popping in and out of existence?

Are virtual particles really constantly popping in and out of existence? Or are they merely a mathematical bookkeeping device for quantum mechanics?

Hi,

Because virtual particles do not exist when you "look at them", but only turn up when you "turn away" they may also be viewed as a quantum bookkeeping mechanism.

Many in the scientific community view them as an ad hoc abstraction that is required because our current view of reality as described by quantum mechanics is not complete.

It is thought by this segment of physicists that virtual particles will no longer be required when a more complete view of quantum mechanics is integrated with general relativity and gravity in general.