Jan Oort:

Jan Oort was born in 1900 in Franeker, Netherlands. In 1932, Oort was observing the motion of the stars in the Milky Way galaxy. He noticed that the stars had a tangential acceleration, yet they were held together like our planets are held towards the sun. Like our sun's mass holds earth from flying out of our solar system, He concluded that there must be a large mass keeping the galaxy from flying apart.

After calculating the tangential velocities of the stars in the milky way, he found that the mass at the center of the galaxy must be at least twice that of all the stars orbiting the center combined. Read More about the discovery here.

 

Find more on Oort's life here.