The librarian found hundreds of thousands of ancient forerunner ships in the LMC. Millions of years ago the forerunners eradicated the precursors in that galaxy, but they never came home out of great shame. Surviving precursors made themselves into the powder stuff but over the ages it became corrupt (or perhaps it was always intended) and thus the flood were born. These are the flood that came to the milky way. It can be assumed the precursors dwelled in many other galaxies, but their existence in the milky way and the LMC is the only thing we know about. Forerunners themselves, after all, may be living in other galaxies as well. A small mention is made in the first foreunner book that forerunners could travel not only to any place they wished, but other dimensions as well. For all that, they were always less advanced than the precursors. So it is likely that precursors (flood) live in other galaxies.