Genome space too vast to search exhaustively

The space of possible genomes is far too large to search comprehensively: even if all 10^88 particles in the observable universe were turned into DNA base pairs and reshuffled a billion times per second for the age of the universe, one could only exhaustively check strands of about 180 base pairs, whereas the human genome is three billion base pairs, which is why natural selection must proceed by random mutation and culling rather than comprehensive search.