The intergalactic medium is matter in the space between the galaxies. Whether there is a genuine, all-pervading IGM, in the same way that there is an interstellar medium within galaxies, is unknown. Certainly, we know, from its X-ray emission, that plenty of hot, tenuous gas resides in rich clusters of galaxies (those containing many galaxies within a relatively compact region of space). But evidence for the existence of intergalactic gas in the Local Group (aside from tidally displaced material such as the Magellanic Stream) is controversial and there is no data yet whether, in the vast inter-cluster spaces, gas just gets thinner and thinner as you go away from one cluster until you reach the far outskirts of the next, or whether there is a true inter-cluster medium. The intergalactic presence of dark matter is also conjectural at this stage.