A single point of infinite density and mass. Blackholes mass is so great that nothing can escape it's gravitational pull, not even light.
Blackholes are supposedly a reality but there is no way (currently) to prove that they are what they are. Blackholes are created by a star or planet of great mass exploding and then collapsing in on itself creating a gravitational pull that just pulls everything in more eventually ending in a blackhole. Blackholes are often identified by circles of light that orbit them because of the gravity. Blackholes also give off X-Ray and
infared signals making them easier to discover.
Blackholes are often theorized as tears in the
time-space continuum meaning they can be portals to other points in space or time. Many sci-fi books and movies theorize blackhole possibilities, such as
L. Ron Hubbard's "Mission Earth" novels using blackholes as an energy source.