The 21 cm radiation is produced when hydrogen, which is a proton and an electron, undergoes a spin-flip transition. This radiation is easy to detect because it is a radio wave, which is transmitted through Earth's atmosphere, and is not scattered easily by gas and dust. The spin that flips is not a true rotation of the proton or electron, it is an intrinsic property of the particle. Astronomers believe that there is a vast amount of dark matter surrounding our galaxy.
Gravity is the force that holds our rotating galaxy together.
If there was not enough mass, which produces the gravity, then stars and other matter would be flung out of the rotating disk. Gravity holds everything in place. Astronomers have measured the rate of rotation of our galaxy. They have also estimated the amount of mass that is found in normal matter such as stars, planets, gas, and dust. They have found far less mass in the normal matter than is required to keep the galaxy together at its rotation rate.