Energy itself is a weirdly defined thing at those sizes. It's sort of a wavy property rather than a physical action.

Vacuum has a lot more energy than its "supposed to" have. This is a huge problem because Einstein says that energy equals mass (multiplied by a constant), so if there is no mass, how can there by energy?

The energy of vacuum is observable, but the mass is not. So the question really is "Where is the rest of this energy?"

The idea of virtual particles is then that the vacuum is full of these energy fluctuations that equals out without ever turning into actual particles with mass.