One of the most fascinating things about the universe is how little we really know. And just like we want to know what happens when we die, science has asked how the universe will end for as long as man has been able to think about such concepts. The truly fascinating thing is just how many theories the scientific community has produced—and how wildly different some of them are.
The most prominent theory for how the universe began is the Big Bang, where all matter first existed as a singularity, an infinitely dense point in the abyss of nothing. Then something caused it to explode. The matter expanded outward at an incredible rate and eventually formed the universe we see today.
The Big Crunch, as you might have guessed, is the Big Bang’s opposite. All that matter expanding outward at the edges of the universe is being affected by our universe’s gravity. According to this theory, gravity will eventually cause this expansion to slow to the point where it halts and begins to contract instead. The contraction will bring all of that material (planets, stars, galaxies, black holes—everything) back to the center until it becomes that infinitely dense singularity again, wiping out everything.
And then we’d be left with the same conditions that the universe had before the Big Bang—all the matter of the universe condensed into an infinitesimal point.
This is, however, unlikely to happen based on current knowledge, since we’ve recently discovered that the universe appears to be expanding at an accelerating rate.