It's beginning, it's composition, it's theoretical ends. (Insanely) Long story made short.
It all started with what we call the Big Bang.
Boom! Everything suddenly appeared, probably erupting from a singularity containing all material substance and thus, all energy.
Matter, anti-matter, energy, everything went from microscopic dimensions to titanic in much less than a second. It was the moment right after the Big Bang: The cosmic inflation.
The universe at the time was subject to enormous pressure. Everything was expanding at ridiculous speeds...
At that moment, it seemed like everything would drift apart infinitely. But this is where our savior, gravity, came in.
The universe's pressure became less intense, as objects were getting further apart, and gravity was able to slow down the fast expansion of the universe.
The objects in the universe are still drifting apart, but in less insane speed; this allows us, as mere humans, to see the visible universe.
At our time, the universe is composed of many things:
Matter: The building blocks of our reality. Composes everything that is visible.
Anti-matter: Disappeared for an unknown reason after the Big Bang. Appears and disappears mysteriously in the void.
Dark matter: Matter that we cannot see. It's existence is theoretical, but almost a fact; despite it's invisibility, gravity and pressure have effects on it, like on normal matter.
Dark energy: Mysterious energy that seems to be accelerating the universe's expansion. According to astrophysicists, there is dark energy everywhere in the universe.
Void: The empty space between instances of matter. It actually isn't empty: Inside it, matter and anti-matter regularly appear from nothing and cancel each other sporadically. Some of the void is full of these events, some isn't, creating a complex soup of dense and less dense spaces.
The universe can end in many possible ways. Three of them are listed here.
The Big Crunch: In this theory, the dark energy in the universe would slow down and eventually stop the universe's expansion. Gravity would take over pressure and cause all things in the universe to slowly drift back towards each other. Then, all things would start accelerating, until everything is brought together in an infinitely small space, like before the big-bang. Then, another Big Bang would occur, and the universe would be reborn.
This is the most popular theory. Another theory is explained by the Big Crunch: The Big Bounce. It is not an end-of-the-universe theory, so it wasn't listed here.
The Big Rip: In this theory, the dark energy in the universe would start to accelerate it's expansion. Eventually, everything would expand at such dramatic speeds that galaxies would start being ripped into pieces, followed by solar systems, suns, planets, and then matter itself would separate. The speed would reach a level at which particles would not be able to support their own self, and then, reality itself would be ripped into pieces.
In this theory, the universe would be a single event, unless if, and only if, many universes exist far apart of each other. Following a Big Rip, for example, the remnants of two universes would meet, and re-create some sort of Big Crunch, followed by a new Big Bang: The birth of another universe.
The theory of the Big Rip is slowly becoming more popular, since recent the discoveries, It seems that dark energy has a tendency to accelerate the universe's expansion in greater proportions than thought before.
The Big Freeze: In this theory, the universe would become stable. But, after billions and billions of years, the stars of the universe would burn off their fuel and die. Then, after another billions and billions of years, all the gas in the universe, the fuel for the stars, would be used, and so the universe would grow colder and colder, darker and darker... The last stars, red dwarfs (stars with the longest life span) would continue shining for another trillion years, until the last of them finally dies. Only black holes would remain almost forever.
Until everything left is forever bound to true emptiness.
Thank you for reading. Please leave me comments and reviews if I forgot anything.