Except, perhaps, under "strange" circumstances. In 1984, physicist Edward Witten hypothesized that at extremely high densities, nuclear matter could melt into its component quarks, remaining stable as a mixture of up, down, and strange quarks. That proposal has led physicists to look for nuggets of strange quark matter in particle accelerators, while astronomers search for quark stars. Have the astronomers found them at last? They really aren't sure. They point out that there are other possible explanations for what they've seen. But they have more motivation than ever to keep looking.