Awesome colloquium today on the basics of dark matter by NYU's David Hogg. Just generally great explanations, but my fave parts were probably when he compared my boss here to a Roomba (dedicated to studying the universe's dust...) and when he described what would happen if two universes with different physical laws collided. "It would be bad. Very bad."
Also, we only know the grav constant, G, within about 5 decimal places. Not that that's too shabby, but we know G*M_sun within 13 decimal places. Because the planets are *awesome* test particles. We can predict the location of say, Mars to within a centimeter. That's pretty crazy!!