The matter of dark matter
From the rotation of our own Milky Way galaxy, to the expansion of the universe, much of the night sky is moving in ways we can’t quite explain. To plug the wholes in our understanding we have dark energy and dark matter. We don’t actually know what they are, but they can still surprise us.
Here’s a bit more about the surface area of a sphere, used to explain the inverse square law.
Here’s a bit of math about acceleration.
Here are tedtalks on dark matter.
Here’s an article about the second galaxy found that seems to have no dark matter in it.
And here’s an article about the 19 dwarf galaxies found that don’t seem to have any dark matter.