The second theory is the result of galaxy interactions. As the galaxies interact, large masses of dark matter may be separated and eventually coalesce into dark dwarf galaxies. This was just a theory until evidence was released in august.  The picture on the bottom shows two galaxy clusters colliding; and as the galaxies collide they are severely distorted. The visible matter interacts with itself and was mapped using the Chandra x-ray observatory and superimposed on the image in red.  Using gravitational lensing the investigators were able to map out the regions of dark matter, in blue.  So what we see it the visible matter slows down due to gravitational drag, while the dark matter continues on unimpeded.  Eventually the large masses of dark matter will separate and become dark galaxies.