| Line | In addition to conventional galaxies, the universe |
| contains very dim galaxies that until recently went |
| unnoticed by astronomers. Possibly as numerous |
| as conventional galaxies, these galaxies have the |
| (5) | same general shape and even the same |
| approximate number of stars 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 |
| (10) | 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. |
| (15) | These galaxies may constitute an answer to the |
| long-standing puzzle of the missing baryonic mass |
| in the universe. Baryons—subatomic particles that |
| are generally protons or neutrons—are the source |
| of stellar, and therefore galactic, luminosity, and so |
| (20) | their numbers can be estimated based on how |
| luminous galaxies are. However, the amount of |
| helium in the universe, as measured by |
| spectroscopy, suggests that there are far more |
| baryons in the universe than estimates based on |
| (25) | galactic luminosity indicate. Astronomers have long |
| speculated that the missing baryonic mass might |
| eventually be discovered in intergalactic space or as |
| some large population of galaxies that are |
| difficult to detect. |