From Scientific American:
For more than 70 years, astronomers, cosmologists and physicists have known that ordinary matter must be surrounded by vast quantities of an invisible substance–not substantial enough to collide with atoms or stars but massive enough to keep galaxies from flying apart. Dubbed dark matter, the mysterious stuff has eluded detection through any means other than its gravitational impact, leading some to propose that Einstein’s general relativity fails to adequately describe how gravity actually works on galactic scales. Now a relatively recent collision of two galaxy clusters has lifted the veil between ordinary and dark matter, proving the latter must exist.
More here. Also in Scientific American, Robert Caldwell answers the question “What are dark matter and dark energy, and how are they affecting the universe?“
