* Gravity: This is the dominant force. The immense mass of the galaxy, including billions of stars, gas, dust, and dark matter, creates a strong gravitational pull that keeps everything bound together. The stars, gas, and dust orbit the galactic center due to this gravitational pull.
* Rotation: The Milky Way is constantly rotating, which helps to counterbalance the inward pull of gravity and prevents the galaxy from collapsing in on itself. Think of it like a spinning ball of yarn – the centrifugal force from the rotation helps keep it from unraveling.
* Dark Matter: While we can't directly observe it, scientists believe that dark matter makes up a significant portion of the galaxy's mass. Its gravity is essential in holding the galaxy together, especially in the outer regions where the visible matter is less dense.
It's important to understand that these forces are in constant interplay. The galaxy is not a static object but is constantly evolving, with stars being born, dying, and moving within the galactic disk. This dynamic equilibrium is what keeps the Milky Way together.
