Galaxies are surrounded by gas clouds extending far beyond their luminous edges.
Galaxies are surrounded by gas clouds extending far beyond their luminous edges. Many stars are extremely dim, and therefore detectable only by gravitational lensing. And some particles interact with matter only via the weak force—but they may still carry mass, and there may be a lot of them. All of these contribute to a quantity whose density determines whether the universe is closed or open. For ten points, name this ominous-sounding two- word phrase.
Galaxies are surrounded by gas clouds extending far beyond their luminous edges.
Dark Matter or Dark Mass