Originally we thought the universe was still expanding because of the immense force of the big bang, however scientists have discovered that rather than the universe slowing down its expansion, the expansion of the universe is actually speeding up. Right now scientists believe that an unknown force named dark energy is responsible for the continual and accelerating expansion of the universe.
Scientists have no idea what this energy is. They know that there is also a substance that they named dark matter which they think might have something to do with it. They know this substance exists because there are insanely dark patches in space that no light at all can get through. For all scientists know there is so much dark energy in the universe that eventually everything gets pulled apart.
At first only galaxies will get pulled apart, but then smaller objects will get pulled apart like planets meaning death to every living thing in the universe. Finally even atoms will get pulled apart meaning the universe will be merely a great mess of protons neutron and electrons. It could even pull those apart causing the universe to merely be full of quarks, but of course, this is just a theory.
Another theory is that there isn’t as much dark energy in the universe as we thought meaning gravity will prove stronger causing the universe to collapse in on itself, galaxy’s collapsing into galaxy’s, then stars crashing into stars, then planets crashing into planets. All the while the universe is getting smaller and smaller until there is nothing left. Some people say the universe will then start expanding again, due to another big bang. If this is true, it is likely that there have been many big bangs (before the one we know and love), and the universe will keep expanding and detracting for eternity. Again, remember this is just a theory and is not necessarily true, it is a theory and a definite possibility so it could be true. Don’t close your mind to it.
If you want more theories of similar style a YouTube channel called kurzgesagt which has many animated theories like this.