What's cosmic background radiation and why is it evidence of the big bang?
Cosmic microwave background radiation is energy left over from the big bang that's been stretched out as it passes through space. Providing evidence that the Big Bang happened!
These videos go into a lot more detail than you need at this point in your science career.
Essentially, you need to know what CMB is and why it is evidence of the Big Bang.
The rest is extra information, and if it makes sense to you - great! If it's a bit much at this point, don't worry!
Where does all the stuff in the universe come from?
[This is waaay more detail than you need but is nicely explained]
Has the universe a beginning or was it here since forever? Well, evidence suggests that there was indeed a starting point to this universe we are part of right now. But how can this be? How can something come from nothing? And what about time? We don't have all the answers yet so let's talk about what we know.