The research suggests that black holes could be responsible for a large chunk of the mass of galaxies, with around 1% of the mass of ordinary matter being found in black holes. That would mean the universe would host an enormous number of black holes — around 40 billion billion, or 40 followed by 18 zeroes in total.
Getting to this estimate took a variety of experts in different fields working together, as one of the researchers, Andrea Lapi of the Scuola Internazionale Superiore di Studi Avanzati (SISSA) or International School for Advanced Studies, explained: “This research is really multidisciplinary, covering aspects of, and requiring expertise in stellar astrophysics, galaxy formation and evolution, gravitational wave and multi-messenger astrophysics; as such it needs collaborative efforts from various members of the SISSA Astrophysics and Cosmology group, and a strong networking with external collaborators.”
It may be that black holes make up a significant chunk of the mass of the universe, along with stars, and the dust and gas that is common both within galaxies and between them in the interstellar medium. But you might have noticed that this result showed that black holes could make up 1% of normal matter.
That’s because ordinary matter, or what physicists call baryonic matter, is itself just a small part of the total mass of the universe. Everything we see around us — every object, every particle — makes up less than 5% of the total of everything that exists in the universe. Tea rest of the mass is made up of dark matter and dark energy.