I am not a particle physicist (black holes and stuff are more up my alley), but here it is. The accepted model is that vacuum is the lowest possible energy state of all fields. And this energy is non-zero. So, in a sense, it has some amplitude corresponding to these fields at each point in space. And it has some energy density. QFT says that this density should be infinite, or at least of the order of the Planck density, which is 10^120 times more than what is currently observed as the dark energy. And this is known as the Hierarchy problem.