In other words we may be well missing the vast majority of galaxies, which are too dim, and hence difficult to observe. Now, galaxies can be dim for two reasons. First, they may be very distant and hence only very little light manages to reach earth. Second, they may be intrinsically dim. The property that characterises how dim a galaxy is, is called surface brightness.
Surface brightness is independent of distance and has units of power per unit area.
Low surface brightness galaxies are not yet understood. They may be formed as such, or they may be results of interactions with other galaxies. What is most amazing is that the largest galaxy known today is a low surface brightness galaxy. It has spiral structure and is almost 25 times larger than our own Milky Way. It is named Malin 1 after the astrophotographer's name, whose technique was used to enhance the image above the background sky.