Scientists are not so naive as to simply think "it is expanding now, therefore it has always been expanding." The main reasons why we think there was a big bang are (http://en.wikipedia.org/wiki/Big_Bang#Observational_evidence):
1) The universe is not just expanding, it's expanding in such a way that the relationship between distance and speed matches up neatly along smooth curves.
2) We can see (in microwave telescopes) the cosmic background radiation just where it should be, at just the frequency that was predicted if there had been a big bang (note the prediction was made in 1948, but the microwaves were not measured until 1965).
3) We can see gas clouds in the far distance (12 billion light years), which we see as they appeared 12 billion years ago, which are made of hydrogen and helium in the proportions that we expect would have been made in the big bang, and without the heavier elements that we think would not have been made in a big bang, and there is no other theory that has been able to explain the proportions of the light elements.
4) The way galaxies and quasars are distributed and the way they appear to have developed over time matches what we think would have happened if there had been a big bang (and rules out other ideas such as a steady-state universe).
You also asked:
You suggest we should be believers in this everything from nothing theory without the least bit of skepticism?
No scientist would suggest that you believe any theory without skepticism. Certainly, be skeptical! But skepticism is not the same as refusing to accept an idea just because it sounds far-fetched. If someone does come up with a better theory (where "better" = "makes predictions that match what we actually observe more closely and more efficiently than other theories"), then by all means, out with the old theory and in with the new. And it's certainly fine to attempt to poke holes in the current theory -- indeed, there is surely a Nobel Prize waiting for the person who proves that there was no big bang! But poking holes in the theory has to be done by either finding out that the theory contains contradictions, or finding that it fails to explain something that we can see happens in reality. One doesn't get the Nobel Prize for saying "that doesn't sound right."