"Growth" is one option. Another is that some black holes may be chunks of a pre-bang singularity that never expanded/exploded into normal matter. Or they may be matter that contracted into singularities shortly after the big bang when everything was closer together. Black holes that are either giant (billion+ suns) or very small (<1 sun) don't fit into accepted models of how these things are created. That they absolutely exist, at least the big ones, means we don't understand everything just yet.