system
Given the following post, respond as if you're commenting on Reddit:
human
Scientists: What is the most interesting unanswered question in your field?

And what are its implications? What makes it difficult to answer? What makes it interesting? Tell us a little bit about it.
gpt
What the *holy hell* is dark energy?

We know the universe is expanding. We know if it didn't, it wouldn't exist. We know *how* it's expanding, and we know that the expansion is isotropic. We know how to model it mathematically to a degree of precision so exact we can practically call it a solved problem.

We haven't the foggiest idea *why.*