|MadSci Network: Astronomy|
Most of space is seen to be empty, very empty. If you were to shrink the entire solar system to be the size of the head of a pin, the nearest star (Proxima Centauri) would be about 15 feet away! So stars are light sources with a lot of mostly empty space between them (there is a bit of gas, mostly hydrogen, between the stars, but not much else). However, just like we think planets tend to be in solar systems, most stars reside in large groups of stars called galaxies. Roughly 300 billion stars make up the galaxy we reside in, the Milky Way (that's roughly 500 stars for every person on Earth!). The space between galaxies is like the space between stars, mostly empty (again, with a little bit of gas possibly floating around between the galaxies). So most of the universe is pretty dark since it is empty and most of the light in the universe, which comes from stars, is therefore concentrated in galaxies. Therefore galaxies are sort of "islands of light" in an otherwise dark and apparently empty universe. They are considered major features of the universe because they are where most of the visible matter in the universe resides. However, before you get your hopes up, it turns out that the visible matter may only be a tiny portion of what is out there! One of the most surprising things astronomers have discovered in the last 20 years is that the vast majority (like 80%) of the matter out there in the universe may in fact be invisible, and so we call it dark matter. So yes, galaxies are the major visible features of the universe, but they may not be where most of the matter in the universe resides!
[Moderator's Note: Another important point is that all the stars and other matter in each galaxy is gravitationally bound together - so the galaxy travels as a unit. While there are larger bound objects in the universe (called clusters and groups of galaxies), they are not nearly so cohesive or well- ordered. This is probably another reason why galaxies are given precedence over other organizations in the universe.]
Try the links in the MadSci Library for more information on Astronomy.