What is dark matter?
space.com: Distinguished University Professor Glenn Starkman, co-chair of the Department of Physics, explained what we know—and don’t know—about dark matter, including the two most prevalent explanations for it: that there is “more stuff [matter] that we don’t see with our telescopes,” or that “Newton’s laws and even [gravity] are wrong on the scale of galaxies and everything bigger.” Starkman explained the latter idea is usually called modified gravity or Modified Newtonian Dynamics.
