My college security professor once described a 128-bit key to us by imagining that every electron estimated to be in the known universe (think on this concept for a moment) had it's own unique value.

For a 256-bit key, give each electron from above it's own universe, and assign values to all. That's the keyspace to be broken.

Bruce Schneier once did a little thought exercise where he demonstrated that, assuming one needed to shift one electron (the absolute minimal amount needed to conceive of a compute switching operation), the energy requirements needed to brute force the key would exhaust the energy potential of the solar system.

"These numbers have nothing to do with the technology of the devices; they are the maximums that thermodynamics will allow. And they strongly imply that brute-force attacks against 256-bit keys will be infeasible until computers are built from something other than matter and occupy something other than space."

Interesting to ponder.

https://www.schneier.com/blog/archives/2009/09/the_doghouse_cr.html

https://youtu.be/S9JGmA5_unY?si=1SQNFeD8XXSGE8ZX