Mass density is a measure of the amount of mass per unit volume of a substance or object. It is typically expressed in units of kilograms per cubic meter (kg/m^3) in the International System of Units (SI).
Inter-galactic gas refers to the gas that exists between galaxies in a cluster. It is primarily made up of hydrogen and helium, with small amounts of other elements such as carbon, oxygen, and nitrogen.
A cluster is a large group of galaxies that are gravitationally bound together. They can range in size from a few dozen to thousands of galaxies.
The typical mass density for inter-galactic gas in a cluster is very low, averaging around 10^-27 kg/m^3. This is much lower than the density of air at sea level on Earth, which is around 1.2 kg/m^3.
The mass density of inter-galactic gas in a cluster is typically measured through observations using telescopes and other instruments. Scientists can also use computer simulations to estimate the mass density based on the known properties of the gas and the cluster as a whole.