the most basic explantation i can put rather simply. ill go ahead and post in more detail on another thread. but anyways we can see how the universe was 10 years ago. it sort of leaves a footprint in space. we can also see how it was 20 years ago and 30 and 40. and we can see how it is now and we can see where it is going. tomorrow it will be bigger than today. yesterday it was smaller. two days ago it was even smaller than that. 1 year ago it was smaller than that and 10 years ago even smaller 20 smaller 30 smaller and 40 smaller. logically since the only force that is making it expand is inertia we can say that a million years ago it was even smaller. think about it and visualize it 20 billion years ago and it was pretty damn small. since there is nothing that would cause it to be a certain size than start expanding for no reason we can say that if we travel back in time the universe will keep getting smaller and smaller. go back 20 billion years and start time it would be super small and then if you could watch it from the begining you would have the big bang, wich scientist would rather call the big stretch or the big expansion. now this is very very simple. the way i explain it here would constitute theory but from all the information we have its beyond a shadow of a doubt or as we put it "fact". ill actually get into it on another thread not sure when maybe the next couple days as thanksgiving is coming and end of term is creeping up on me.. and yes hawkings was against the theory but that was before nasa sent up COBE wich got us alot, and i mean alot of info on the universe. the reason hawkings was against the big bang was because we didnt know of any laws of physics that would allow such a dense area of matter to expand the way it did. but we do now. also nasa just sent another probe called MAP earlier this month like on the 12th to follow up on what cobe found. it will not start doing any readings however till like 2002. and hopefully we can see the shape of the universe the size and where its going, not to mention alot of other cosmology that doesnt have to do with the universe's origin.