In the name of God , most gracious most merciful
On that day We shall roll up the heavens like a scroll rolled up for books. As We originated the first creation, so We shall bring it forth again. It is a promise (binding) upon Us. Truly We shall fulfill it (as We promised it). (21:104)
Big Crunch teory
The Big Crunch is one scenario for the end of the universe, and it will result if the universe has a spherical geometry. This "spherical geometry" is not an abstract idea: It actually relates to what the shape of the universe would be if one could observe it from the "outside."
In this case, the universe contains enough mass - it is above the critical density - to actually stop its expansion. Once it stops expanding, it will start to contract. Slowly at first, and then faster and faster, the universe will contract and galaxies will come closer to each other. Eventually, everything will merge, for the universe will no longer be large enough for separate galaxies or stars. As it continues to shrink, the universe will heat to huge temperatures, and everything will be compacted into a black hole. Finally, at the end, the universe will be as it began - an infinitely small, infinitely dense, and infinitely hot point. No one knows what, if anything, would happen after that *.
An easy way to think of this is by throwing a ball; you throw a ball up into the air. Your release is like the Big Bang, and starts the ball's acceleration. As the ball climbs skyward, it slows its ascent because the Earth has enough gravity to slow it down and pull it back to it. This is like the mass of the universe being enough to overcome its expansion. As the ball reaches its maximum height, it stops, which is what the universe will do if it is over the critical density. Then, ever so slowly, the ball begins to fall back down, faster and faster, until it reaches your hand again (unless you miss). This is the end of the ball's throw, and is like the end of the universe.