The big bang theory provides a complete description for the evolution of the Universe that
is in reasonable agreement with all data. But
some features of the big bang are less than satisfying because they must be assumed
without solid justification. New theories have been
developed that attempt to explain the "why" of some of the big bang theory's assumptions.
They retain the big bang, but modify the description of its earliest instants and go by the name of
"cosmic inflation".