Calculating the number of atoms in the universe post-Big Bang is complex, as atoms only became stable after the recombination epoch, approximately 380,000 years later. The observable universe has an average density of about 0.25 atoms per cubic meter, contributing to roughly 4.9% of the total energy density according to the Lambda-CDM model. The remaining energy density consists of dark matter, dark energy, neutrinos, and electromagnetic radiation. Estimating a total number of atoms is challenging due to the potential infinite extent of the universe. Thus, while density can be measured, the total number of atoms remains an elusive figure.
