Gravity holds the universe together by keeping the large bodies that make up the universe together. It keeps the Earth in orbit around the sun, the sun in orbit around the center of the Milky Way and the Milky Way bound to other nearby galaxies known as the Local Group.
Gravity is the force that causes any two objects to be drawn toward each other. People experience gravity every day. When a book falls or a ball is thrown, it comes back to Earth because of gravity. Without gravity, everything on Earth would float off into space. The same thing is true of large bodies in the universe. Gravity acts as a sort of cosmic glue that holds large bodies together.
There are still some mysteries when it comes to gravity and the role it plays in holding the universe together, however. Scientists have calculated that there is not enough mass in clusters of galaxies to hold them together and believe that dark matter, or matter that it not visible using any type of telescope, is responsible for creating the cosmic glue that holds them together. Scientists currently estimate that as much as 90 percent of the matter in the universe is dark matter.