The theories of QED suggest that the universe is full of “virtual particles,” which are not really particles at all. They are fluctuations in quantum fields that have most of the same properties as particles, except they appear and vanish all the time. Scientists predicted the existence of virtual particles some 80 years ago, but we have never had experimental evidence of this process until now.

Source: Scientists identifiy the first evidence of ‘virtual particles’ hopping in and out of existence, which were first predicted in the 1930s | It’s Interesting