But it really isn't a head scratcher, and it's not a wild guess, and also, dark matter is not the same as dark energy.

Think about some random person born in the 1950s. It will be impossible for you to directly prove that that person has two parents, but with all your knowledge about biology, and your knowledge that humans could not be cloned in the 1950s, it's not a head scratcher, unless our understanding of biology is completely off, it's not just speculation there were two parents. That's the comparable situation with dark matter. It's not a head scratcher. Every indication shows there is a lot of mass around that doesn't interact with light.