When did “you weren’t there” become a valid argument against something for which there’s ample evidence? Why are religious people still using this worn-out, ridiculous meme to try and disprove the Big Bang, abiogenesis or evolution? Let me break down why I, personally, think it’s (I’m not going to mince words) stupid.
You weren’t there when your grandparents were born, yet you accept it as fact because your very existence testifies to the event necessarily occurring at some point. This, of course, isn’t direct and verifiable evidence because all you have is (possibly) a paper trail and word-of-mouth testimony.
You weren’t there when your parents were born, yet you accept it as fact for the same reasons as above. You accept that there’s sufficient evidence to produce a working explanation of your descent through your parents and grandparents and you really don’t feel the need to question it a whole lot. You’d never really consider arguing with your parents about these things, using the “you weren’t there” rebuttal, would you?
What would happen if you found evidence that shook the foundations of your knowledge as to your origin? What if you were adopted or conceived via artificial insemination? In this case you’d be mistaken that your parents are actually your parents. Would you re-evaluate the evidence and adjust your understanding/beliefs to fit the facts as you know them? Would you do more research to understand why you were originally mistaken? It makes sense that you would.
Having used the “you weren’t there” argument against scientific concept x, why are you doggedly arguing for a god’s creation of the universe, or the Great Flood, or the crucifixion or Armageddon? You realize that you weren’t there, right? You further realize that the authors of the Bible weren’t there either, right? How is it that your rebuttal “works” against science but not against your own unfounded beliefs? Seriously, what’s the deal with that?
Since your creation myths and outlandish tales of huge, supernatural miracles that left no trace behind seem so implausible — and you’ve been given massive amounts of evidence to explain how these things have come about (just do a search and see for yourself!) — why would you not re-evaluate your beliefs and adjust accordingly? I’m not saying you have to blindly accept whatever some scientist(s) says, but you can’t declare yourself informed while ignoring everything that contradicts what you believe. And you certainly can’t justify using “you weren’t there” to refute scientific theories that have withstood harsh scrutiny from the global scientific community and a barrage of purposely ignorant fundies.
Educate yourself! Learn something! Open your eyes to facts!