Well, I was disappointed by most of the answers.
First, the hypothesis/theory *was* first proposed by a Roman Catholic monk/scientist named Lemaitre. Look it up and read about it - it's important. He believed that the competing theories were wrong to exclude the possibility of creation as described in Genesis, and so he developed his own theory of how the universe came to be in its present state that
a) fitted all known evidence
b) also agreed entirely with the biblical creation account
It's interesting that at the time the "Steady State Theory" (SST) was the popular theory among physicists. Lemaitre's theory was scornfully and publicly rejected by most physicists of the time, including Einstein. The reasons? First, it was clearly biblically-based. Second: there was no evidence directly supporting the Big Bang Theory (BBT) in favor of the SST. Understand, though: the BBT agreed with the available evidence just as well as did the SST.
When Hubble discovered that pretty much everything in the universe was moving - and moving apart - this shattered the SST and the opposition to the BBT. This is where astronomy and mathematics first came in in a way that put the BBT ahead of all the other theories (there were others than the BBT and SST, of course). Einstein actually had the integrity to publicly apologize for his earlier remarks deriding the BBT. You'll have to look up Hubble's Law if you want to know more about the mathematics and astronomical observations involved (both are part of Hubble's Law).
Regarding theory - a theory is an idea, but it is an idea that scientifically explains some phenomenon (one answer said "an observed fact" - which is close enough). Theories are considered valid as theories, scientifically, if the available evidence does not contradict the theory. A hypothesis is a scientific explanation of a phenomenon that doesn't really have any supporting evidence yet. A hypothesis is just an idea. A theory is an idea that has been tested scientifically to some degree (even if its a very small degree) and found to be a possibly correct explanation of the phenomena. So, normally in science we have 3 stages
* hypothesis - the untested idea that explains some phenomenon
* theory - the idea has been tested at least a little by scientific experiment and agrees with the results
* law - the idea has been proven to be true conclusively. Every prediction made by the theory regarding the nature of the phenomenon has been validated by experimental results
One answer said,
"A theory in science is as close to fact as science will get. "
This is an obviously false statement - though it is true that some scientific theories are not verifiable with our current level of technology. In other words, it is true that many scientific theories are in no apparent danger of becoming anything more than theories.
Another thing that I would like to point out: scientific theories, if both successful and popular, grow many "legs". Take the BBT, for example: all the BBT itself says is that at some point the universe came into being through some sort of explosion. That's it.
Now, give a world of astrophysicists a century to work with that, and all sorts of "additions" are made to the BBT. There's a dozen different theories regarding how long ago this happened, a dozen different theories regarding how it happened, why it happened, what is going to happen, etc. All of these "legs" are (unfortunately) considered to be a "part" of the BBT - even though each and every one of these variations of the BBT has one or several variations with which it competes. So, in other words we have the "basic" theory, and then we have all of the competing variations of the BBT that include the characteristics that I mentioned and many more.
This variations are where most of the heavy-duty math and astrophysics enter. By measuring things like how fast cosmic entities (stars, galaxies, etc.) are moving apart, scientists can estimate how long ago the BB occurred. By estimating how much mass and energy exist in our universe, scientists can estimate things like
* how powerful was the explosion
* will things keep moving apart forever, or is there enough matter in the universe that eventually everything will stop and then fall back in on itself (the Oscillating Universe Theory)
etc. etc.
Yes, a majority of scientists *accept* this general theory - meaning that they think it is more likely to be true than competing theories. (I think that "agree with" is too strong a phrase.) I don't think that any particular variation of the theory has a majority of support from scientists.
Jim, http://www.bible-reviews.com
P.S. A fellow on Y!A with far more knowledge of the origins of the BBT than I had the kindness to correct some of my statements.
First, the scientist who originated the BBT, LeMaitre, employed quite a bit of math to the Theory of Relativity in a way that had not been done before. "...it included a new and unique solution to the equations of General Relativity".
Second, one of the remarkable things included in his theory was a "crude approximation of the Hubble Constant".
In other words, the original BBT included a mathematical basis that was, at the time, unique to the BBT, and explained at least some phenomena that were not as well explained by the SST.
Finally, the word "explosion" was poorly chosen (I hesitated over it when writing, in fact). This user kindly corrected me: the proper word is "expansion".