Recent findings in astrophysics suggest that the observable universe can be modeled as a sphere of radius r = 13.7 × 109 light-years = 13.0 × 1025 m with an average total mass density of about 1 × 10-26 kg/m3. only about 4% of total mass is due to "ordinary" matter (such as protons, neutrons, and electrons). part a estimate how much ordinary matter (in kg) there is in the observable universe.