Something out there holds swarms of galaxies together and keeps their stars from flying apart, but scientists still haven’t learned what this invisible substance is. Known as dark matter, it gathers to form a colossal cosmic scaffolding. Astronomers believe that galaxies formed at the densest points in this weblike structure, and the dark matter continues to hold them in place with its gravity. Its bulky presence can be detected by tracking stars on the outskirts of galaxies, which move at speeds that would be impossible if only visible matter – a galaxy’s other stars and gas – were pulling them. Astronomers have also mapped this unseen substance with the help of an effect predicted by Einstein’s general relativity; Dark matter’s gravity wrinkles space-time, bending light rays as they pass. Such measurements indicate that dark matter could make up 90 percent of the universe’s total mass. These days, cosmologists are searching for the identity of dark matter, trying to detect the elusive substance responsible for arranging everything we see in the sky.
(National Geographic, May 2005)