It is charged with studying the halo of hot gas surrounding the Milky Way as part of a mission to search for matter missing from the universe. The $3.7 million satellite is designed to perform a big scientific mission — studying X-rays from the gigantic halo of hot gas surrounding the Milky Way galaxy. Those measurements could help explain what happened to a third of the mass from the early universe, which seems today to be missing. \"When the universe was young, the matter was all in the same form, so it was easy to measure,\" he says. \"We're pretty bad at observing hot gas,\" he explains.