Dark matter is stuff in the universe that has gravity but we don't know anything about it. Together, dark matter and dark energy make up 95% of the universe; the other 5% is other things such as us, trees, planets and galaxies.
This makes up 27% of the universe alone and means it is possible for galaxies to exist. We know more about what it isn't than what it is but we do know that it interacts with gravity.
Dark energy is even more of a mystery than dark matter but makes up 68% of the universe. Considering all the galaxies and other things only make up 5%, that is a lot! In fact, dark energy and dark matter put together are 19 times as much as everything else. Dark energy is stronger than any other energy. One theory is that temporary particles constantly form from nothing and disappear again. The energy that this creates could be dark energy.
This shows just how little we know about the universe.