Looks like there is now some hard evidence for dark matter. More commentary and explanation here. (Apparently it doesn't rule out MOND, but it does confirm that dark matter exists, which has been one of the most embarrassing questions in physics for a couple of decades now — given that visible matter accounts for only about 4% of the energy density of the universe, where is the rest hiding?
The two big question marks in our knowledge of what the universe is made of are dark matter (which doesn't interact with the type of matter we're familiar with, but which clumps gravitationally) and dark energy, which I can't get my head around (how the hell does something have negative pressure?). Dark matter appears to account for 22% of the universe, with dark energy making up the other 70-something percent (darn it, why is the universe seven-tenths made up of something I don't understand even the layman's definition of?). And today it looks like we're down one question mark.