Virtual particles are indeed real particles. Quantum theory predicts that every particle spends some time as a combination of other particles in all possible ways. These predictions are very well understood and tested.

Quantum mechanics allows, and indeed requires, temporary violations of conservation of energy, so one particle can become a pair of heavier particles (the so-called virtual particles), which quickly rejoin into the original particle as if they had never been there. If that were all that occurred we would still be confident that it was a real effect because it is an intrinsic part of quantum mechanics, which is extremely well tested, and is a complete and tightly woven theory--if any part of it were wrong the whole structure would collapse.

www.scientificamerican.com...

These fascinating variations in the appearance of energy, which sometimes manifests itself as light, sometimes as matter, have stimulated the distinguished French physicist Auger to exclaim enthusiastically, in a monograph on cosmic radiation: "Who has said that there is no poetry in modern, exact and complicated science? Consider only the twin-birth of two quick and lively electrons of both kinds when an overenergetic light quantum brushes too closely against an atom of matter! And think of their death together when, tired out and slow, they meet once again and fuse, sending out into space as their last breath two identical grains of light, which fly off carrying their souls of energy!"
nobelprize.org...

"Another way of thinking about these things is to imagine that all of space, even empty space, is awash with particles, that nature in her infinite wisdom can provide. This is not a metaphor. One of the implications of quantum theory is that these particles do in fact pop in and out of existence in the void. The particles,. . . are all temporary. They are created and then quickly disappear --- a bazaar of seething activity."

web.rollins.edu...