Galaxy Cluster Simulator

Galaxy clusters are the largest gravitationally bound structures in the universe — hundreds to thousands of galaxies, plus a vast reservoir of million-degree intracluster gas, all held together by gravity. This simulator builds a cluster from scratch and lets you tune how many galaxies it has and how fast they're moving (their velocity dispersion), then compares two numbers side by side: the visible mass you can actually see in starlight and hot gas, and the much larger virial mass the virial theorem (2K + U = 0) says the cluster needs to keep its fast galaxies from escaping. Toggle dark matter off and watch the cluster fly apart in real time — a direct, interactive demonstration of the mass discrepancy that first convinced astronomers dark matter exists at the scale of entire clusters, not just single galaxies.