Interactions between particles don't produce energy, but there is an uncertainty relation between energy and time that means that over a short-enough time perio

Interactions between particles don't produce energy, but there is an uncertainty relation between energy and time that means that over a short-enough time period the energy of a system is uncertain, and this can be thought of as allowing for virtual particles to appear "out of nothing" for a brief time. This uncertainty in energy over small time-scales provides a "floor" for the energy of empty space.