They are detected mainly with radio telescopes but traces of them can sometimes be seen with optical telescopes. The key problem is determining how far away they are. They were the first objects to form in our local part of the Universe. They are galaxy-making Lego, if you like. Professor Blitz and his colleagues have made one key assumption which sets their work apart from earlier models of galaxy formation.
Nobody really knows what 'dark matter' is.
This model has been very successful in reproducing the properties of the clouds that we actually observe. There is still a small but steady infall of the high-speed clouds into our Galaxy. Clouds like these in deep extragalactic space may hold the key to another long-standing mystery. It's been thought for a long time that these absorbers must be some kind of intergalactic cloud. It looked like these clouds could be attached to the outer parts of galaxies or free-floating.