I came to Less Wrong as a firm atheist, but realized that I did not seem to have any personal basis for this belief. After I read this article, I decided to take a serious look at possibilities for a deity.

Funny. I came to LessWrong as a firm atheist, then realized that atheism was an answer to a question nobody ever had any rational right to ask.

Therefore, to the best of my knowledge science has no rational explanation for the amount of matter in this universe.

In other words, "science doesn't know how X happened, therefore god did it." This is the classic argument from ignorance. Much better to say "I don't know why matter predominates." That is a perfectly legitimate answer, even in the unlikely event that NO scientific explanation is ever forthcoming.

Obviously, the Bayesian probability for this is very weak, but it is also the only theory that I see that has any chance of being true, so it is one that I, as a rationalist should believe in.

There are a few things that are confused here. First of all, if this explanation is the only one you can think of, that does not mean you should believe in it.

If I watch a magic trick and I can't figure out how it was done, the only 'explanation' I can think of is magic. That does not mean it is rational to believe it was magic until I think of a better explanation. I just admit my confusion and keep thinking. :)

I do not expect this theory to last, or even be true, but it has the highest probability of being correct.

Actually, what "highest probability of being correct" MEANS is precisely that you should expect this theory to be true.