In the first half of the last Century, scientists realised that there must be more to space than meets the eye: without some invisible force hanging on to them, clusters of stars rotating around galaxies ought to be being flung out into space like children letting go on a playground roundabout. That force, they knew, must be gravity, but its origin - where it was coming from - no one knew.A popular theory at the time was that millions of small stars we couldn't see were lending their mass to the equation, but by carefully logging what was out there in our own Milky Way Galaxy, Gerry Gilmore... Like this podcast? Please help us by supporting the Naked Scientists