The vacuum between the stars is not empty. The interstellar medium consists of dust and gas, which in turn can be in atomic, molecular and ionized form. Its density varies widely. Interestingly, it is greatest in cool, dense regions where matter is mainly in molecular form and one could count up to 1 million molecules per cubic centimeter. In hot, diffuse regions, on the other hand, matter is mainly ionized and one finds only a single ion per 10,000 cubic centimeters. Compared to, say, the capabilities of a vacuum chamber constructed by humans, with still ten billion particles per…