Learn more physics!
This article says that we have discovered super massive black holes with a mass of billion of suns. Can there be black holes out there massier than a whole galaxy? Those hyper massive black hole, could they be an explanation for dark matter? And with such a discovery, do we have to reconsider the total mass on the universe? Because we based our calculation with visible matter (stars, galaxies). What about invisible matter such as hyper massive black holes? http://www.space.com/13819-black-holes-largest-massive.html
You ask a very timely question! Only in the past ten years or so have
astrophysicists realized that most if not all galaxies have supermassive black holes at their centers. And our galaxy is no exception! How and why this is true is one of the hottest topics in the study of galaxy formation. Many questions remain open, but several important results are already clear.
Perhaps most importantly and most surprisingly, the mass of a galaxy's central black hole is found to be related to the mass of the galaxy. More precisely, the black hole mass is correlated with the velocities and mass of the spherical components of stars ("stellar bulge" or "spheroid") at the centers of galaxies. Roughly speaking, a galaxy's central black hole generally is found to be about 10-3 times the spheroid mass, or about 0.1% of the spheroid mass.
The discovery that black hole mass is a small but roughly constant multiple of galaxy mass has several important implications. First, it tells us that the formation and growth of supermassive black holes is somehow tied up with the formation and growth of galaxies. If this does not seem surprising, it is important to keep in mind that even the most massive black holes (for example, those just discovered) have a "size" (Schwarzschild radius) about the size of the solar system, and thus more than 10,000 times smaller than their host galaxies. Yet somehow this mismatch of tiny and huge objects seem to grow together.
Second, since supermassive black holes are generally found to be a small fraction of their host galaxy's mass, they are not the dominant source of gravity in a galaxy. The black holes' masses, large thought they are, are too small and just as importantly too centrally concentrated to explain the large and widespread gravity source that is implied by galaxy motions. Thus supermassive black holes cannot be the dark matter in galaxies.
Also, because supermassive black holes grow together with galaxies, then when two galaxies collide and merge, we expect their two supermassive black holes to merge as well. Indeed, we hope to detect the gravitational radiation from these black hole mergers.
There are even a few systems where we see two black holes in colliding galaxies! This also means that it might be possible that when two galaxies merge, one supermassive black hole could be ejected from the system. If that were to happen, then the "lonely" ejected black hole might be the sort of thing you suggest at the beginning of your question.
(published on 12/11/11)
Follow-up on this answer.