My background is in Health Physics and as such, I have had only rudimentary instruction in quantum mechanics, so my understanding (such as it is) is largely conceptual. With that in mind, this may be a very ignorant question, so I apologize in advance. I understand the theoretical basis for vacuum fluctuations and I understand the implications like the Casimir effect and Hawking radiation. However, one large piece of my understanding is missing. In simple terms, an electron, for example, may interact with a positronium virtual particle, which will very quickly annihilate. This is a "virtual" interaction and can only be detected indirectly by the effects mentioned above and others. So far, so good. My question is - what happens to the annihilation photons? Shouldn't the universe be awash with 511 keV photons from virtual positronium annihilation events? Clearly it is not, and the fault is with my understanding of vacuum fluctuations. Thanks.