In addition to the reference mentioned above:
Virtual particles are theoretical and have never been observed [not experimentally verified].
For some reason I haven't figured out, people think virtual photons play a part in near field antenna radiation.
Here is a closely realated article:
I'll excerpt just a few key ideas:
...Hawking Radiation from another source.....(also not directly observable)
(I know Wikipedia also discusses Hawking Radiation)
Hawking's mathematical development of Hawking Radiation does not explicitly involve virtual particles; he used virtual particles as an intuitive description of how radiation might originate.