If planet X were indeed a black hole, how would we know? The researchers argue that dark-matter annihilation is the thing to search for. No one knows if primordial black holes exist.

Among the possibilities cited in the paper are a dark, icy world that’s more than 100 AU away, or a super-Earth that’s about 300 AU away, or a super-cool brown dwarf that’s out in the Oort ...

For decades astronomers have searched for a possible “Planet X” in the far outer reaches of our solar system, speculating that something big and dark may be lurking out there, its ...

The paper reported discovery of what appears to be a dwarf planet, dubbed 2012 VP113 (nicknamed “Biden”), that's currently about three times farther from the sun than is Pluto -- out beyond ...

Dark matter is far more abundant in the universe than the "normal" stuff we can see and touch. Learn about the evidence for dark matter in the fourth episode of "We Don't Planet." Skip to main content ...