system
Given the following post, respond as if you're commenting on Reddit:
human
Why does vacuum-energy exist? How can nothing have some energy?

Or maybe I'm just very confused.
gpt
Well any theory that predicts a non-zero vacuum energy also predicts that the vacuum itself has a complex non-trivial structure.  Like the vacuum of quantum electrodynamics for example.  It can fluctuate, it can interact, it can become polarized.

It is also worth pointing out that from this perspective the vacuum isn't "nothing" but rather the situation where all quantum fields are in their lowest energy state.   These vacuum states of fields have both energy and fluctuations.