Collisions between gas clouds in such a merger can trigger the clouds to collapse and form stars. High-speed collisions do not lead to mergers, but instead result in the gas and dust being swept out of the galaxies. By assuming that a cluster of galaxies is gravitationally bound and then measuring the speeds of galaxies in the cluster, the mass needed to hold the cluster together can be determined. When this is done, we notice there is ten times more mass in a cluster then contained in the visible parts of the galaxies. Determining where the rest of the mass is and what it is composed of is the missing mass problem.
Gravitational attraction causes galaxies to cluster.
An organic molecule is one involving carbon. The latter indicates that these molecules are present in the debris that orbits in our solar system. The Miller-Urey experiment simulated primordial conditions on Earth. In modern versions of this experiment, the gases carbon dioxide, water, nitrogen, and hydrogen are sealed in a container. An electric arc is used to simulate lightning.