Black holes are among the most mysterious objects in the universe, making them the subject of much research, discussion and science fiction. Recent theories have posited that every black hole contains a universe — and that we’re inside a black hole right now — and that the universe itself started when a four-dimensional star collapsed into a black hole.

There are some things we do know about these cosmic oddities. Taking advantage of the Black Friday spirit, NASA and other space agencies celebrated Black Hole Friday last week. We thought we’d get it on the action ourselves, with nine facts about these enigmas. Plus, the slideshow above highlights just how beatiful these things can be.

1. The gravitational pull of a black hole can greatly slow down time itself, according to relativity. If you could take a spaceship to a black hole, orbit around it for awhile, and then fly back to Earth, you would have successfully traveled to the future.

2. Some equations suggest that every black hole contains a universe — which would mean our universe is inside a black hole right now.

3. While black holes are most definitely real, they have theoretical opposites called white holes, which would endlessly spew matter into the universe. They were thought to be purely hypothetical, but an unusual gamma ray burst observed in 2006 is turning out to be a potential candidate for a real-life white hole.

4. Supermassive black holes likely exist at the centers of most galaxies. And since galaxies sometimes collide, that means black holes do too, and when that happens, it’s thought that one black hole ‘kicks’ the other out of the galaxy.

5. Black holes are black because their gravity is so strong that not even light can escape. But they do emit radiation, usually called Hawking Radiation, after Stephen Hawking, who first theorized its existence.

6. The Milky Way has a supermassive black hole in its center, and it seems to have exploded about 2 million years ago in an event known as a Seyfert flare. The radiation from the black hole would have been 100 million times more powerful than it is now; the explosion may have even been visible from Earth.

7. Black holes can emit material at nearly the speed of light. Using an array of radio telescopes, a team of scientists looked at a galaxy 1.5 billion light-years from Earth and found a black hole doing just that. The jet is so powerful that it’s blowing gas right out of the galaxy.

8. Black holes are the densest objects in existence. If you made a black hole with the mass of the entire Earth, the black hole would be 9 millimeters across.

9. Black holes can form when stars collapse in on themselves after death. They keep growing by eating the dust and gas around them. No one’s really sure how the biggest ones, called supermassive black holes, are born.

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