The discoveries in physics that inform current dark matter theory, everything from Newtonian gravity to Einstein's relativity to the Higgs boson, are examined in this authoritative guide. It examines the biggest question facing physicists today: if we are only seeing about four percent of matter in the universe, what is the rest of it made of? The lengths that scientists are going to find what they call dark matter are revealed. Alternative theories to dark matter are also discussed, as are the scientists who don't believe that dark matter exists and are conducting their own experiments to disprove its existence.