LOL, your use of 'reckon.' But I reckon you reckoned wrong. Ditto your other use of 'reckon.' They did much more than "reckon" how much gravity it would take to keep the stars together, and the outer stars orbiting just as fast as the stars in the central region of the galaxy.
Did they go there and set up instruments to measure gravity and orbiting stars?
Or did they stay on earth and "reckon" all their estimates?