Black holes, the densest objects in the universe, are quite difficult to spot as they absorb everything that’s in close proximity, including light. However, a group of researchers from Italy and the UK used a simulation called SEVN, or Stellar EVolution N-body code, to work out the amount of mass "hidden" in black holes and estimated the number of existing black holes in the universe.

The research, which took a variety of experts in different fields working together, suggested 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.

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. That's because ordinary matter, or baryonic matter, is itself just a small part of the total mass of the universe.

Everything we see around us, including every object and every particle, makes up less than 5% of the total of everything that exists in the universe. The rest of the mass is made up of dark matter — a theoretical particle that makes up around 27% of the universe — and dark energy, a theoretical form of energy that scientists think must exist due to the way the universe expands.