Scientists have long known that the hydrogen gas in galaxy clusters is searingly hot — about 10 million degrees Kelvin, or roughly the same temperature as the center of the sun — which is so hot that hydrogen atoms cannot exist. Instead the gas is a plasma consisting of protons and electrons. But a puzzle persists: There is no straightforward explanation for why or how the gas stays so hot. According to the normal rules of physics, it should have cooled within the age of the universe. But it hasn’t. Scientists have created conditions similar to the hot gas in gigantic galaxy clusters.