Dark matter and energy are theories that are a little more complicated than that, in that astronomers can observe gravitational clustering more powerful than the regular matter allows for; dark energy is beyond the areas of my understanding. So far it seems to me the best guess is Shut up and Calculate.

Funny to see how perfectly dark matter is used as a mysterious answer, in the sense of "How could the universe be expanding? Dark matter!"

I thought the mysterious answer to that particular question was 'dark energy'. I don't think dark matter is enough to (not particularly) explain it.