The mysterious force that may be causing the expansion of the universe to accelerate is called dark energy.

Here's why it's called "dark":

* We can't see it: Dark energy doesn't interact with light, so we can't observe it directly.

* We know it's there: We know it exists because of its gravitational effects on the expansion of the universe.

We don't fully understand dark energy, but it's a fascinating and important topic in cosmology.