The Big Bang
I love how it is all explained in this series, especially The Big Bang. I've been a supporter of the theory for quite some time now but after having it explained in this way with all the evidence shown here in the Atom series... I no longer consider it to be a theory, to me its a fact :)
Just needed to get that out there... this is a great series and if you like it you should probably check out the BBC's Visions of the Future (Could be called 2057, I'm not sure. Its with Michio Kaku I think), its very interesting and does bring a sense of hope for the future of not only mankind but for our planet and its other inhabitants as well. Science is the way to go!