What is the ratio of mass to vacuum in the universe?
Very low. We have an average of about 0.27 hydrogen atoms/m3. “The observable universe contains about 3 to 7 × 10^22 stars (30 to 70 sextillion stars), organized in more than 80 billion galaxies, which themselves form clusters and superclusters. Two back-of-the-envelope calculations give the number of atoms in the observable universe to be around 10^80. 1. Observations of the cosmic microwave background from the Wilkinson Microwave Anisotropy Probe suggest that the spatial curvature of the universe is very close to zero, which in current cosmological models implies that the value of the density parameter must be very close to a certain critical value. This works out to 9.9×10^−27 kilograms/meter^3, which would be equal to about 5.9 hydrogen atoms per cubic meter. Analysis of the WMAP results suggests that only about 4.6% of the critical density is in the form of normal atoms, while 23% is thought to be made of cold dark matter and 72% is thought to be dark energy, so this leaves 0.27 h
