Scientists now understand that supermassive black holes lurk at the heart of most, if not all, galaxies. These cosmic titans have masses millions and even billions of times that of the sun, yet tremendous size doesn't constitute a problem when supermassive black holes are seen in the local universe and thus more recent in cosmic history.
Supermassive black holes become an issue, however, when they are seen in the early universe, and they already have masses equivalent to billions of suns. That is because there must be some mechanism that allows supermassive black holes to rapidly gather mass and grow to such giant sizes, yet all the existing mechanisms for this growth suggest this process proceeds too slowly for objects like this to exist just after the Big Bang.
