If you're interested in cosmology, this is an excellent article about the expansion of the universe. It is probably the best explanation I've seen about dark energy. We still don't know what it is, but the key insight for me is that it's inherent to space itself. As more space is created by expansion, the total amount of dark energy will increase, even though the energy density of dark energy remains constant. That explains why expansion started increasing again about 6 billion years ago, which I always found confusing.

https://bigthink.com/starts-with-a-bang/what-drives-expansion-universe/