Transcript: There’s a basic problem in cosmology. Astronomers measure the light emitted by different objects, but they really want to detect or measure the mass. So the measurement of the mass is indirect. Some galaxies for example are so diffuse that their light level falls below the sky brightness, and so they are missing from galaxy catalogs yet they may contain much dark matter. The intergalactic medium is so diffuse that it cannot be seen in optical light. It is hot and should emit x-rays or ultraviolet radiation, but this has not been directly detected yet. So astronomers must use a variety of techniques to infer the mass density of the universe from the light emission.