as a common type of conventional galaxy, the spiral, but tend to be much larger. Because these galaxies ’mass is spread out over larger areas, they have far fewer stars per unit volume than do conventional galaxies. Apparently these low-surface-brightness galaxies, as they are called, take much longer than conventional galaxies to condense their primordial gas and convert it to stars—that is, they evolve much more slowly.

These galaxies may constitute an answer to the long-standing puzzle of the missing

baryonic mass in the universe. Baryons—subatomic