The ancient Greek philosophers were right; everything that the universe contains is reduced to four types of elements. Two of them are well known: photons of light and ordinary matter, with which stars, galaxies, planets and ourselves are built. The other two represent a mystery. On the one hand we have dark matter, of which there is 10 times more than ordinary matter. It is a totally unknown type of matter that we know is there because of its gravitational effect on galaxies. The other is dark energy, which represents 70% of the matter-energy density of the universe and not only is it not very well known what it is, it is also not understood what it does there. As if it were a sequel to Star Wars, these two representatives of the dark side are the ones fighting for control of the future universe: dark matter, through gravity, tends to slow down the expansion, while dark energy tends to speed it up.
So what will happen to everything in the universe? Until within 3,000 million years, nothing remarkable. But then our neighboring and almost twin galaxy, the Andromeda Nebula, will collide with the Milky Way. The result will be a bright new irregular galaxy, with rivers of stars flowing outwards and clouds of gas compressing to eventually form new stars. After 2,000 million years, the result of the collision of these spirals will be visible: an elliptical galaxy.
The collision with the Andromeda galaxy will have a spectacular climax. The center of the Milky Way, occupied by a black hole 3 million times the mass of the Sun, will merge with Andromeda’s, which is 10 times larger. In doing so, a burst of X-rays and gamma rays will sweep away all the planets and possible life still on them, and a train of gravitational waves will hit each star like the clapper of a bell.
Little by little, the stars of the galaxy will go out after exhausting their nuclear fuel. The heaviest will go first and then the lightest. The last light to come out of the new Milky Way will be red. Although no one will be able to see it. The accelerated expansion will have caused the 100,000 million galaxies that we can see today through our telescopes to be lost behind the cosmic horizon. This is so because the time that the light would need to reach us will be greater than the time elapsed since the creation of the universe. In other words, the galaxies will have moved so far away that their light will not have time to reach us. The last galaxy will be out of sight in 150 billion years.
Occasionally a faint flicker of light will flood the galaxy, when two brown dwarfs collide or a black hole swallows some drifting stellar debris. Within 100 billion years, our once luminous cosmic city will be populated by the corpses of what were brilliant suns: gray dwarfs, neutron stars and black holes.
Around some of them will revolve the remains of their planetary systems, although not for long: in a trillion years the planets will abandon their orbits around their dead star. The universe will have become a cold and dark place. If we look at the sky a hundred trillion years later, we might witness a slow diaspora of dead stars into the outer void, a sign that the galaxy is breaking apart due to occasional close encounters between stars. This happens rarely today, but given enough time, even the rare becomes commonplace. Planetary systems will have disappeared, either because they have fled away from their central star or because, like the remains of our Earth, they end up falling into it after a long spiral journey.
Our visible universe will be made up of runaway stars and the black hole at the galactic center surrounded by a cloud of dark matter. Black holes will serve as cosmic sweepers, gobbling up the few stellar corpses that swarm through space. Then, with nothing else to put in their mouths, they will start devouring each other. And, as always, the biggest will have the upper hand. The last remnants of what was our known universe will be huge super black holes, which will also have devoured all the dark matter around them. In 10,000 million quintillion quintillion quintillion years these will evaporate, leaving behind a trail of radiation that will cool as the universe continues its inexorable expansion towards the Great Solitude, practically empty of everything we know. Only subatomic debris from earlier, long-forgotten eras will remain.