Observable universe
Matter content—number of atoms

Assuming the mass of ordinary matter is about 1.45 × 10 kg as discussed above, and assuming all atoms are hydrogen atoms (which are about 74% of all atoms in the Milky Way by mass), the estimated total number of atoms in the observable universe is obtained by dividing the mass of ordinary matter by the mass of a hydrogen atom. The result is approximately 10 hydrogen atoms, also known as the Eddington number.
detectable_content:postscript