A majority are. Not all, clearly, but definitely a very strong majority. My guestimate would be in the range of 80%-95% (big error bars because obviously my personal experience isn't a terribly good judge...but I haven't personally met a single cosmologist who isn't an atheist, though I have met an astronomer who isn't).
My basic position is that the concept of a god falls into just a couple of categories, depending upon the definition a person uses for the word:
1. Specific definitions: the more specific the definition of a god becomes, the more unreasonable it becomes. Either because it's just more complex, and thus less likely without evidence, or because it starts to contradict itself (e.g. the problem of evil), or because it starts to contradict simple observation of reality. These gods cannot exist, in essence.
2. Vague definitions: in order to avoid being ruled out by simple observation, many attempt to clothe their god in vagueness. But in doing so, they essentially define their god out of existence: what they end up describing may as well be a dumb, unthinking, unfeeling law of nature (such as the 1/r^2 falloff of gravity or some such). To call such a thing "god" seems, to me, a bit ludicrous (though many pantheists would disagree, I suppose). But more to the point, the definitions become so vague that there is no way to determine what these people actually mean by the word in the first place, and so they may as well be saying that snargle slumphs exist (and then not bothering to go to the trouble of stating what a "snargle slumph" actually is).
I have never seen anybody come up with a definition of a god that avoids these two. And even if they did, then there's the next problem: lack of evidence. So yeah, there's just no reason to believe in any god or gods.