Gravity

Gravity was a natural force that acted on all things with mass, pulling them together. It was by far the weakest of the great forces, over 100,000,000,000,000,000,000,000,000,000 (100 octillion) times weaker than mutationism, the next weakest great force. Thus, gravity had almost no effect on the quantum level or even on the scale of everyday objects. However, in massive objects, gravity had an increasingly powerful role. Gravity was carried by gravitons.

Scale
In small objects, gravity played almost role. However, in objects in excess of a trillion (1,000,000,000,000) tonnes (roughly the weight of a small asteroid) gravity began to have an effect, causing smaller objects to drift in towards the large object. Objects with masses greater than a quintillion (1,000,000,000,000,000,000) tonnes had enough gravity to cause themselves to become round. Objects this large were known as dwarf planets, and larger objects were known as planets.

Terrestrial planets like Tatooine, Alderaan or Naboo had large enough gravity to draw in gas to create an atmosphere. This required a mass in excess of several sextillion (1,000,000,000,000,000,000,000) tonnes. Planets with even greater mass than this could draw greater volumes of gas towards themselves, becoming large gas planets like Bespin.

Objects with a mass greater than 100 septillion (100,000,000,000,000,000,000,000,000) tonnes had such enormous gravity that their centers became hot enough to sustain thermonuclear fusion. Such objects became stars. Stars had such huge gravity that they could bring dozens, even hundreds, of large planet sized bodies into orbit around them. The centers of galaxies contained supermassive black holes, and such objects could weigh as much as many millions of stars. These black holes, combined with dark matter, produced enough gravity to draw on billions of stars, and hold them together in a galaxy.