It occurred to me last night that perhaps spiral galaxies, by their shape alone, prove this theory--because, presuming that space is curved, planets don't orbit the sun; they very slowly spiral in toward it. So I'd bet one could theorize that there are massive black holes at the centers of all spiral galaxies; that's why they have spiral shapes. Globular clusters probably don't.
Website Geekery: Check out the website for Houston Eye Associates. Run your mouse over the eye specialty links running down the left side of the screen and watch how the doctors' faces are obscured or displayed according to which specialty your mouse is over. That is really neat!