It's no secret that space is full of mysteries.
Our species has barely gone past our own planet's moon, and only one of our probes, Voyager 1, has even left the solar system. Much of what we've learned about deep space has been pieced together from falling objects and views from telescopes.
Some of the famous (and chilling) space mysteries, like the face on Mars (it was shadows), or the black knight satellite "UFO" (it's satellite debris), have been solved.
But space is rife with unexplained phenomena that put those two mere optical illusions to shame.
And some of the seven included in this slideshow could hold the key to understanding the universe.
DON'T MISS: 7 space mysteries no scientist can explain
Black holes
Black holes are the ultimate cosmic quicksand. They're formed when a giant star collapses, imploding into a tiny area of such intense gravity, even the surrounding light is sucked in.
This means that although we've got a sense of how black holes work, we've still never actually seen one — they're invisible to telescopes that pick up electromagnetic radiation, light, or X-rays. We can only guess what they look like on the inside.
The Giant Void
Unlike a black hole, the Giant Void isn’t a hole in space — instead, it’s curiously empty of both matter and dark matter. And also different from a black hole, light can pass through the void, though scientists believe it contains dark energy.
It’s not the only void in space, either, although it is the largest, with an estimated diameter of 1.3 billion light years.
Dark Matter
Dark matter is still a mystery, but we're relying on it to help explain some of the unknowns of our universe — cosmologists believe as much as 27% of the universe is dark matter.
We're more certain of what dark matter isn't rather than what it is. It's not made of black holes (the light warping that they'd cause isn't present).
One theory: the dark matter of the universe is made up of primordial black holes.
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