Dark Matter's discovery raises the question of what exactly is its nature? Is it the well-known but elusive neutrino, or is it some exotic particle not yet discovered. So far this question is unanswered. But cosmologists have been able to determine one important property of dark matter. It is fairly well determined that the dark matter is "cold," i.e. moving at a speed small compared to the speed of light, as opposed to "hot," moving at a speed comparable to the speed of light. Cosmologists have discovered that dark matter is cold by observing the imprint that it leaves when galaxies begin to form around it in the very early universe. That imprint remains on the Large Scale Structure seen in the distribution of galaxies today.

You can compare the pattern of Cold Dark Matter (CDM) clusters with that of Hot Dark Matter (HDM) by downloading this simulator (requires the free Wolfram CDF Player). See if the number and size of the voids in the CDM simulation (left) more closely match the sample of galaxies from the Sloan Digital Sky Survey (SDSS) plotted than the HDM simulation (right). See how varying the "fractionFilled" and "depth" parameters change the simulation structures. (The CDF player may take a couple of minutes to load, and "angularSize" values of 64 and 128 take considerably longer to process.)

South Carolina State University, 01/24/2015