From what we see in space around us, it appears that everything at great distance is moving away from us, and thus that the universe is expanding. If we could run the whole process in reverse, it appears that everything would come back together a a single point. The sudden expansion from this point is called the Big Bang.
How do scientists know about the Big Bang, and in what detail do we know? The following videos should help. Pay particular attention to the evidence that supports the strange notion of the Big Bang, and the lack of any evidence (up to now) to call the notion into question. Not all the details are worked out however. There could still be some surprises in store.
Watch These Videos
The narrator of Crash Course Astronomy, Phil Plait, is a fast talker, and some of the illustrations appear and disappear quickly. But remember, it's a video, and you can rerun it when he's lost you. If you press your left-cursor key, either with the video running or paused, you back up five seconds. Just press it repeatedly until you've backed up far enough to get a replay. During replay, pause to see figures longer. Phil has a time limit of about 10 minutes.
You don't.
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For a literary take on the Big Bang, read the short story"All At One Point", by Italo Calvino, HERE. You will have to scroll down to the story, with the help of the Contents table at the top.
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Here's more from Crash Course Astronomy on the Big Bang and the future of our universe.
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