The universe is every single shit that exists everywhere, and perhaps even some stuff that don't quite exist but kinda do in a way.
Beware, it's hot
This theory basically says the whole universe is just the consequence of a hot explosion. There are many theories about where the fuck such explosion could have come from including the one that says it came from the collapse of a previous universe created by a previous explosion, which presumably was proven flawless after the brilliant guy that came up with it repeated the loop a couple thousand times. The most accepted theory nowadays is the original and complex one that it came from nowhere. Some of our folks at the Large Hadron Collider are actually trying to recreate the big bang by simply smashing shit together, so they know how it was like, instants before fucking the whole actual universe and dying a horrible death. We really wish them luck.
Fuck you, LHC
So basically, we know shit about the big bang
We could say the end is up to us but thats utter bullshit: If it happens while we're still alive it will most likely we wont see it coming and be instantly carbonized, squished by gravity and/or get irradiated and explode in the goriest way possible. It has recently been proven that simply observating stuff changes it's behavior (insert geeky science site link none will read anyway here) so we can't know how it will in fact end, just accept the fact it's gonna be painful.
Some geniuses came up with original the idea that it's gonna be like the big bang, only backwards. Duh