Physics is both deeply inspiring and deeply humbling, in a way that few other disciplines are today. On the one hand, physicists have found the elusive Higgs boson, the particle responsible for mass. On the other hand, physicists are comparatively clueless when it comes to dark matter, which makes up the vast majority of the Universe.

Comparatively clueless, that is to say, but not completely clueless.

In the video below, Joel Primack, an astrophysicist at the University of California-Santa Cruz, explains that we don't yet know the real nature of the dark matter "beyond that it’s pretty cold." Cold refers to the terminology that Primack coined in 1983. Dark matter is either hot, warm or cold "depending on how rapidly it’s moving in the early stages of the Big Bang," Primack says.

Hot represents nearly the speed of light. Cold means "it’s moving so slowly that its motion hardly matters," Primack explains, while warm is an intermediate case.  

Since we know that dark matter is cold, Primack says this is enough to allow us to predict "in great detail the large scale structure of the universe, the organization of the galaxies and to some extent the satellites of the galaxies."

This is also enough information to make Primack hopeful that we’re about to cross a threshold "and have a complete understanding of the origin and the evolution of the universe." 

Watch the video here:

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