Pressure differences near 0 deg K

I'm thinking of a void between one galactic cluster and the next and wondering if, although very sparsely populated, whether a region like this can be considered a gas (and use the gas laws - Boyle and Charles... )?

The behavior of real gases usually agrees with the predictions of the ideal gas equation to within 5% at normal temperatures and pressures. At low temperatures or high pressures, real gases deviate significantly from ideal gas behavior. In 1873, while searching for a way to link the behavior of liquids and gases, the Dutch physicist Johannes van der Waals developed an explanation for these deviations and an equation that was able to fit the behavior of real gases over a much wider range of pressures.

Gas laws are useful when there is enough matter to matter. The molecules have to be close enough to interact with each other.

My understanding is that the conditions you describe are too sparse for gas laws to be used.