If we chose to shovel matter along a straight line through the observable Universe, what is the average mass of matter per unit area that our shovel will collect? To answer this question, I did a simple calculation before my morning jog at sunrise today.
The answer depends on how far the shovel goes. Projecting all the matter out to the farthest galaxy, MoM-z14, discovered last month by the Webb telescope at a cosmic redshift 14.44 or equivalently 280 million years after the Big Bang, the answer is about 0.5 grams per square centimeter, of order the mass per unit area of a thumb. This establishes the cosmic rule-of-thumb: the observable universe yields on average as much mass per unit area as a thumb.
This mass budget includes mostly dark matter whose nature is unknown. Ordinary matter accounts for only 16% of the total budget or 0.08 grams per square centimeter out to MoM-z14, of order the surface mass density of a fingernail.