A Black Hole
Now, you figure that matter has to go somewhere, and it (theoretically) is ejected at a white hole. A white hole is described as "a postulated celestial body that is the time reversal of a black hole. While a black hole acts as a point mass that attracts and absorbs any nearby matter, a white hole acts as a point mass that repels or (perhaps) even ejects matter." Now, there is no proof of white holes, and it is doubtful you could pass through one (see wormhole, because of a paper published in 1962 by John A. Wheeler and Robert W. Fuller proving that Schwarzschild wormholes are disconnected instantaneously after they are formed, not allowing even light to pass through). I've said waaaaay too much, so I'm going to shut up now. Have a nice day.
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.
Joe: But I hear scientists are attempting to create bla-- *Earth is sucked into blackhole in 3 seconds*