In the late 90's, scientists began the Two-degree-Field Galaxy Redshift Survey, or the 2df survey, in an effort to do what Bryan Boyle, a scientist working on that project, described as "weighing the universe". Although that is a very simplified summary of the survey, the results were clear; our visible universe does not have the mass that it has to have. The universe was much lighter than we expected- too light to be curved, and only containing a third of the mass necessary to be flat. That was a point where the existence of dark matter was proved to be necessary. Either the laws of physics and mathematics were wrong, or there was some invisible, atom-less mass that enabled the universe to exist as we know it.
