Source: Chandra@University of Texas
Astronomical observations show that everything that we have ever seen, including stars and planets, is a small fraction of what there is in the Universe. At least 90% of the Universe is made of something else - of dark matter (DM). We know that it exists only because its gravity pulls on the things we see. But it emits no known form of radiation, so we do not know what it is made of. Many possibilities have been proposed, including elementary particles left over from the Big Bang, underluminous or dead stars, and million-solar-mass black holes. The problem of DM - what it is and how it affects galaxy formation and evolution - is one of the most fundamental puzzles of astronomy.
Smaller galaxies are observed to be more dominated by dark matter. The smallest galaxies known are at least 99 % dark. These galaxies look incredibly gossamer, but they are really like cannonballs: they contain a much higher density of dark matter than do giant galaxies. These galaxies did not know when they formed that we would be able to discover them 10 billion years later only if they managed to hold onto 1 % of their mass in stars. Instead, when their first stars died in supernova explosions, they may in many cases have blown away so much of the remaining gas that too few stars were ever formed for us to find the empty halos that are left.
Smaller galaxies are also more numerous; tiny dSph galaxies outnumber large galaxies like our Milky Way. More of them continue to be discovered; clearly we have not found all of them. Since almost-dark galaxies are the most common ones known, darker galaxies may be more common still. (read source)