Do all galaxies have black holes at their centers? If not, what provides
the gravity to hold the galaxy together?
Joseph L. Heil
According to computer models and theory, it is not necessary for a galaxy to have a black hole at its center. The mass of the stars and gas clouds in the galaxy attracts and this gravitational attraction holds many galaxies together. This said, many, and probably most galaxies do have massive black holes at their center, including ellipsoidal galaxies and spiral galaxies with central bulges.
Rob Landolfi, Science Teacher, Washington, DC
'Physics is mathematical not because we know so much about the physical world, but because we know so little; it is only its mathematical properties that we can discover.'