Intensity mapping is the measurement of cosmic matter density, in particular, the density of matter through time and space. This helps lay out the history of the universe and its parameters. It is referred to as line intensity mapping as generally it involves mapping the intensity of one or more spectral lines. Common are surveys such as 21cm Experiments.
Lyman-alpha forest-based surveys, both of which are examples of hydrogen intensity mapping.
Molecular lines are also of interest, such as carbon monoxide lines, e.g., to map out potential star-forming regions over time and space.