>>16728511
40 billion marbles represent terrestrial planets in the habitable zones, assuming 10% stars have one

Sun has 3 planets and 1 dwarf planet in the habitable zone so Mars, Venus and Moon should all have advanced alien civilizations, right?
Self-evidently, no.
"goldilocks zone" around star is just 1 out of dozens of requirements to have a planet habitable for life

consider also other basic things like
>solar stability
>orbital stability
>no neighboring supernovas
>gravity
>atmosphere
>hydrosphere
>geological activity
>asteroid impact frequency
>the cosmic improbability of metabolism developing by random chance abiotically

if you assume these factors also have 10% chance to be "just right", then your 40 billion marbles instantly becomes 400 marbles
Each civilization would have 100 million star systems to explore before running into one another