system
Given the following post, respond as if you're commenting on Reddit:
human
Can someone explain to me the Big Bang Theory LI5?
gpt
When we look at the universe around us, we notice that everything seems to be moving away from each other.  Imagine if you drew a few dots in marker on a balloon and started to blow it up, you'd notice every point moving away from every other point as the balloon expands.  This is exactly what space is doing, it seems to be expanding.

So a bunch of scientists got to thinking, if everything is expanding away from each other, what if we ran time backwards?  Where is everything expanding FROM?  Is there a point at which the entire universe was collapsed into a single point that exploded and caused everything to expand outward?

An explosion generates a lot of heat and energy when it goes off and we even think we've detected some of the heat from the giant big bang explosion in something called Cosmic Microwave Background, which is really just another way of saying heat.

Also, by looking at how fast everything is moving away, we can easily see how far we need to rewind for everything to be in the same place, 13.7 billion years.  This is how we arrive at the age of the universe.

EDIT: Spelling.