Anyhow, extraordinary evidence really just means real evidence. It just needs to be evidence that can be verified and actually supports the theory. Where did life come from? The current favorite hypothesis is abiogenesis, which while not fully explored or understood, seems at this time to be the most promising solution. If you would like to know more, I suggest beginning at Wikipedia and moving on to other sources. Of course my presuppositions will allow unbiased examination of the evidence. That is what Science is all about. Maybe you guys don't understand, but Scientists aren't really that interested in constantly quashing your beliefs, they are interested in the truth, and, by and large, will constantly strive towards it. Even I don't really care about you being wrong, I care about people understanding reality as best we can. As for the criteria for Extraordinary evidence, of course its reasonable, it is, as Crystal Shards said, what has gotten us as far as we are today.
Anyhow, Black Hole point was addressed by Black Knight, so see below.
Things might exist despite having no proof, but that is no excuse to just guess wildly. When we don't know something, it does not mean you are free to make up whatever you want to put in that place, it means WE DON'T KNOW. Now, if God just had no evidence to support his existence, then I would say that there simply was no reason to believe he existed. But as has been proven many times, there is actually evidence AGAINST the existence of God.
Pretty much this.
Science isn't going "Hey, this would be awesome, let's look for a way too prove it exists!", it's going "Wow, I wonder why this shit is happening? Let us search for an explanation."