They are very rare and have two components: what is known as a high surface brightness disk galaxy, with an extended low surface brightness disk surrounding it. (carnegiescience.edu)
Thus far, there have been about a dozen Giant Low Surface Brightness galaxies found, but none more extreme in size than Malin 1, which now has a rival in UGC 1382. (carnegiescience.edu)
Giant Low Surface Brightness galaxies are among the most massive and isolated spiral galaxies known. (carnegiescience.edu)