In transitive terms the difference between expand and boom
is that
expand is to express (something) at length and/or in detail while
boom is to make something boom.
In intransitive terms the difference between expand and boom
is that
expand is to feel generous or optimistic while
boom is to be prosperous.
As a noun boom is
a low-pitched, resonant sound, such as of an explosion.
As an interjection boom is
used to suggest the sound of an explosion.