The Big Bang is not just a theory
http://www.astro.ucla.edu/~wright/cosmo-religion.html

"Popular usage equates theory with an untested hypothesis. But
scientific theories are hypotheses that have passed many empirical
tests. However, even a very well-tested theory can be falsified, as
shown by Newtonian gravity. Falsifying a well-tested theory is the
biggest achievement that a scientist can make, and thus many scientists
are drawn to testing and retesting the best established theories. But
these scientists recognize that the probability of falsifying a
well-tested theory is low, and thus their work may come to naught. One
can encourage the testing of well-established theories by proposing
alternatives theories that explain all known facts but make different
predictions for new experiments. In the 1950's the Big Bang was not
well-tested, and the Steady State model was a very useful alternative
that encouraged many observations. Now the Big Bang picture is well
established, but the inflationary scenario added on before events in
the classic Big Bang is not well-tested. Many alternatives versions of
inflation have been suggested. But do not think that these alternatives
represent weaknesses of the Big Bang".