In the vast garden of the universe, the heaviest black holes grew from seeds. Nourished by the gas and dust they consumed, or by merging with other dense objects, these seeds grew in size and heft to form the centers of galaxies, such as our own Milky Way. But unlike in the realm of plants, the seeds of giant black holes must have been black holes, too. And no one has ever found these seeds—yet.
Source:
https://phys.org/news/2019-09-black-hole-seeds-cosmic-garden.html