They used the word "waterfall". Our solar system, and all the other star systems at the same galactic radius, are swimming up a waterfall. Objects with higher density, like neutron stars and black holes, drift downward, while lighter objects and particles, like nebulae and neutrinos, are flung upward.

Inventing new words is important when dreaming up new theories. A small change in word usage will lead to different logical conclusions. As an example, consider the following two premises:

"Nothing can escape from a black hole, not even light."

and,

"Nothing can escape from a black hole, not even heat."

The implications of one are entirely different from the other and depend heavily on word usage. In the frontiers of physics, words must be carefully chosen if they are to express the deeper meaning of the theory.