Krauss has returned the favor by sharing his knowledge of physics in six books for the public, including the best-selling The Physics of Star Trek . Understanding one requires understanding the other. By studying how gravity affects galaxies, astronomers know that every galaxy must contain more mass than we can see.
Scientists have several theories about the nature of dark matter.
One theory says that dark matter is a type of subatomic particle that we have not yet observed. If dark matter were a new type of particle, then it should affect the way atoms work. On the other hand, this particle would have mass, so each particle would create gravitational force. Because there is so much dark matter, all these tiny forces taken together would affect the structure of the whole universe. Krauss looks for evidence at both scales.