Nice question. Probably a question that that science can’t answer (except to say the big bang) right now, though they’re working on it – though I’ll also defer to Chris on this one.
Failing that, is there an “I’m a Philospher Get me out of here” website? 😉
I’m afraid that is what I’m going to say – the big bang.
There was a competing theory of continuous creation (Hoyle) which is neat and doesn’t need us to try and get our minds round a start point before which there was no time or space. It doesn’t fit observations as well as BB though.
Now I’m going to cheat here and point you at an answer that Evan gave a while ago. at http://ias.im/44.670 Evan was a Postgraduate student here at Jodrell Bank until last year – now he works in Bonn. He’s writing some good essays to answer these questions.