Virtual Particles:

According to quantum electrodynamics (QED), the vacuum is filled with electron-positron fields. Real electron-positron pairs are created when energetic photons, represented by the electromagnetic field, interact with these fields. Virtual electron-positron pairs, however, can also exist for minute durations (less than a Planck time), as dictated by Heisenberg's uncertainty principle, and this at first led to fundamental difficulties with QED.

Excerpt from the Encyclopedia Britannica without permission.