What goes in, must come out.

This simple truism usually alludes to items stuffed into bodily orifices, but it turns out that nature's most extreme orifices — black holes — are no exception.

Black holes, of course, are regions of spacetime where gravity is so strong that not even light can escape their pull. When matter falls into one, it gets crunched into a point so dense that none of our theories even know how to describe what happens to it there. If there's anywhere in the cosmos devoid of an exit, it's within the gullet of a black hole.

Or so we used to think.

A growing number of astrophysicists are now taking seriously the idea that black holes might actually have a way out, a place where the stuff that's swallowed up by them gets spewed back out: a so-called "white hole," reports New Scientist.

White holes are basically black holes in reverse. Whereas a black hole has an event horizon that, if crossed, represents a point of no return, a white hole also has a horizon that inversely marks the point at which there can be no approach. A white hole's horizon is so repellent that not even light can enter.

Moreover, black holes and white holes are inverses of each other in time. A white hole is essentially a black hole's future, and a black hole is a white hole's past. They're the exact opposites of one another in almost every way.

Emphasis: Almost every way. There's one little problem with the theory of white holes: no one has ever seen one before. Which is weird, considering they ought to be among the most luminous objects in the universe, due to all of the energy spewing out of them. Black holes are impossible to see, and yet we now know the universe is teeming with them. White holes, by contrast, should be beacons of light in the night sky. And yet, nada.

In the mystery lie solutions

That's enough reason for many astrophysicists to remain skeptical. Though, astrophysicists were once skeptical about the existence of black holes too.

One reason to keep the faith in white holes is that they're theoretically convenient. The theoretical possibility of white holes is actually predicted by Einstein's general relativity. In fact, white holes are exact solutions to the equations of the theory.

So if white holes existed, they would help us to explain many of the mysteries that still exist about black holes. For instance, they would solve the so-called black hole information paradox — we don't expect information to be lost in nature, and yet if time comes to an end at the heart of a black hole, as we currently theorize that it does, information must be lost.

If white holes existed, information would come bouncing back out. Problem solved.

White holes could even help to enlighten us about the biggest mystery of all, the origin of the cosmos. They would provide an alternative model to the Big Bang, suggesting instead that our universe may have bounced out of a previous collapsing phase of a reverse universe. We'd be on the white hole end of a once-massive black hole.

It's mind-blowing stuff. Until we can detect one of these white holes, however, they're likely to remain mere theoretical curiosities.

There are some candidates. For instance, scientists have recently detected mysterious fast radio bursts coming from deep in the universe, which so far have eluded a consensus explanation. It's possible that these powerful bursts are rays from white holes. That's just a guess at this point, but it's a potential lead.

The only way to truly find out is to keep looking up.

You've heard of black holes, but what about white holes?
White holes that spew matter out rather than suck it in might explain the biggest mysteries of the cosmos.