Density and Composition of the Universe
Based on what we can see, stars fall short of
providing the critical density by a factor of 200. Neutrinos don’t seem to
help. Indeed, the fraction of helium observed implies that matter is a factor of
20 short. But there are good theoretical reasons to believe the curvature
is flat. Dark matter provides about a third of what is needed.
What’s in the
Universe:
VACUUM
Counting particles in 100 sq meters
1 heavy (more than O) atom
100 atoms of C,N,O
100,000 atoms of helium
1,000,000 atoms of hydrogen
30 times that mass in dark matter
(particles of
unknown mass)?
100,000,000,000,000 cosmic photons and as many cosmic neutrinos