The stars will burn out fairly soon, and fairly soon after that, no new stars will be born, and the only things left will be black holes, or the remains of stars, space dust, and dark energy, orbiting or falling into these black holes and black hole galaxies and black hole galaxy clusters. The difference between stars and black holes is pretty simple. While a star exists, it converts mass into energy, sending out heat and light energy far into the universe, while slowly losing the mass that makes it up through this radiation. The black hole does the exact opposite, it sucks in energy, and mass, and light, and dark energy, and anything that exists, which adds to it's mass and gravity, and nothing can escape it, at least not until what in my view is the end result, all the limited propulsion sources pushing the expansion which are remnants of the big bang and the result of stars burning, hot energy, get sucked into these black holes, there's no propulsion left, and gravity is the only force remaining. The gravity of all the energy in the universe, in the form of mass, including what scientists estimate to be the vast majority of the energy in the universe, dark energy, will cause the collapse, however gradually, as while gravity may be weak, with no opposing force pushing the expansion, it will slow, and then slow some more, and be pulled around in an oval shaped orbit, and these black hole galaxy clusters or whatever they look like when they finally start falling back down will smash together all the energy in the universe as fast and hard as it possibly can, causing the big bang, resetting the second law of thermodynamics to the start, and making another similar universe to this one, which in my view must be an endless cycle, or it never could have begun.
Source(s):
What goes up, must come down, every action has an equal and opposite reaction, energy equals mass multiplied by the speed of light squared, and mass equals energy divided by the speed of light squared, energy cannot be created or destroyed it only changes form.
